feat: full file context + patterns-repo support

Major improvements to review quality:

1. Full file context: fetch complete content of all modified files from
   the PR branch and include as reference. This eliminates false-positive
   "missing import" findings since the model sees the entire file.

2. Patterns repo: new --patterns-repo / PATTERNS_REPO flag fetches
   language idiom files from a separate Gitea repo (e.g. rodin/elixir-patterns)
   and includes them as review criteria.

3. Multi-file patterns: --patterns-files / PATTERNS_FILES accepts
   comma-separated file paths to fetch from the patterns repo.

New API methods:
- GetPullRequestFiles: list changed files in a PR
- GetFileContentRef: fetch file content from a specific branch/ref

Prompt changes:
- BuildSystemPrompt now accepts (conventions, patterns)
- BuildUserPrompt now accepts fileContext parameter
- File context displayed before diff for model reference
- Patterns presented as "review criteria" in system prompt

Composite action updated with patterns-repo and patterns-files inputs.

review/prompt.go

@@ -6,15 +6,14 @@ import (
 )
 
 // BuildSystemPrompt constructs the system prompt for the LLM reviewer.
-func BuildSystemPrompt(conventions string) string {
+func BuildSystemPrompt(conventions, patterns string) string {
 	var sb strings.Builder
 
 	sb.WriteString("You are an expert code reviewer. Review the provided pull request diff carefully.\n\n")
-	sb.WriteString("IMPORTANT CONTEXT:\n")
-	sb.WriteString("- You are reviewing a DIFF, not the complete file. Code not shown in the diff already exists in the repository.\n")
-	sb.WriteString("- Imports, type definitions, functions, and other declarations that do not appear in the diff are already present in the file.\n")
-	sb.WriteString("- Do NOT flag missing imports, missing type definitions, or undefined references unless the diff itself introduces a new usage without a corresponding addition in the same diff.\n")
-	sb.WriteString("- Only flag issues with code that is actually being ADDED or MODIFIED in this diff.\n\n")
+	sb.WriteString("CONTEXT:\n")
+	sb.WriteString("- You will receive the full content of modified files for reference, followed by the diff showing what changed.\n")
+	sb.WriteString("- The diff shows ONLY what was added/removed. The full file content provides complete context.\n")
+	sb.WriteString("- Focus your review on the CHANGES (the diff), using the full files for context.\n\n")
 	sb.WriteString("Your task:\n")
 	sb.WriteString("1. Review the diff for correctness, idiomatic code, potential bugs, and design issues.\n")
 	sb.WriteString("2. Consider the CI status — if CI has failed, that is an automatic REQUEST_CHANGES regardless of code quality.\n")
@@ -40,17 +39,20 @@ func BuildSystemPrompt(conventions string) string {
 	sb.WriteString("- Be thorough but fair. Don't nitpick style unless it impacts readability significantly.\n")
 	sb.WriteString("- Line numbers should reference the new file line numbers from the diff headers.\n")
 	sb.WriteString("- If the diff is empty or trivial (only formatting/whitespace), APPROVE with no findings.\n")
-	sb.WriteString("- Never flag 'missing imports' or 'undefined' errors for symbols that could exist in the unchanged portions of the file.\n")
+
+	if patterns != "" {
+		sb.WriteString(fmt.Sprintf("\n\n## Language Patterns & Idioms\n\nUse the following patterns as review criteria. Code that violates these established patterns is a finding:\n\n%s\n", patterns))
+	}
 
 	if conventions != "" {
-		sb.WriteString(fmt.Sprintf("\n\nThe repository has the following coding conventions that should be respected:\n\n%s\n", conventions))
+		sb.WriteString(fmt.Sprintf("\n\n## Repository Conventions\n\nThe repository has the following coding conventions that must be respected:\n\n%s\n", conventions))
 	}
 
 	return sb.String()
 }
 
 // BuildUserPrompt constructs the user message with PR context.
-func BuildUserPrompt(title, description, diff string, ciPassed bool, ciDetails string) string {
+func BuildUserPrompt(title, description, diff, fileContext string, ciPassed bool, ciDetails string) string {
 	var sb strings.Builder
 
 	sb.WriteString(fmt.Sprintf("## Pull Request: %s\n\n", title))
@@ -69,7 +71,13 @@ func BuildUserPrompt(title, description, diff string, ciPassed bool, ciDetails s
 		sb.WriteString(fmt.Sprintf("CI Details: %s\n", ciDetails))
 	}
 
-	sb.WriteString("\n### Diff\n\n")
+	if fileContext != "" {
+		sb.WriteString("\n### Full File Context (modified files)\n\n")
+		sb.WriteString(fileContext)
+		sb.WriteString("\n")
+	}
+
+	sb.WriteString("\n### Diff (changes to review)\n\n")
 	sb.WriteString("```diff\n")
 	sb.WriteString(diff)
 	sb.WriteString("\n```\n")

review/prompt_test.go

@@ -6,7 +6,7 @@ import (
 )
 
 func TestBuildSystemPrompt_NoConventions(t *testing.T) {
-	prompt := BuildSystemPrompt("")
+	prompt := BuildSystemPrompt("", "")
 
 	if !strings.Contains(prompt, "expert code reviewer") {
 		t.Error("expected system prompt to mention code reviewer role")
@@ -18,7 +18,7 @@ func TestBuildSystemPrompt_NoConventions(t *testing.T) {
 
 func TestBuildSystemPrompt_WithConventions(t *testing.T) {
 	conventions := "- Use stdlib only\n- No panics\n"
-	prompt := BuildSystemPrompt(conventions)
+	prompt := BuildSystemPrompt(conventions, "")
 
 	if !strings.Contains(prompt, "coding conventions") {
 		t.Error("expected conventions section")
@@ -29,7 +29,7 @@ func TestBuildSystemPrompt_WithConventions(t *testing.T) {
 }
 
 func TestBuildUserPrompt_Basic(t *testing.T) {
-	prompt := BuildUserPrompt("Fix bug", "Fixes the crash", "diff content here", true, "all checks passed")
+	prompt := BuildUserPrompt("Fix bug", "Fixes the crash", "diff content here", "", true, "all checks passed")
 
 	if !strings.Contains(prompt, "Fix bug") {
 		t.Error("expected PR title")
@@ -46,7 +46,7 @@ func TestBuildUserPrompt_Basic(t *testing.T) {
 }
 
 func TestBuildUserPrompt_CIFailed(t *testing.T) {
-	prompt := BuildUserPrompt("Add tests", "", "some diff", false, "lint: failed")
+	prompt := BuildUserPrompt("Add tests", "", "some diff", "", false, "lint: failed")
 
 	if !strings.Contains(prompt, "FAILED") {
 		t.Error("expected CI status FAILED")
@@ -57,7 +57,7 @@ func TestBuildUserPrompt_CIFailed(t *testing.T) {
 }
 
 func TestBuildUserPrompt_NoDescription(t *testing.T) {
-	prompt := BuildUserPrompt("Quick fix", "", "diff", true, "")
+	prompt := BuildUserPrompt("Quick fix", "", "diff", "", true, "")
 
 	if strings.Contains(prompt, "### Description") {
 		t.Error("should not contain Description header when body is empty")
@@ -66,7 +66,7 @@ func TestBuildUserPrompt_NoDescription(t *testing.T) {
 
 func TestBuildUserPrompt_DiffIncluded(t *testing.T) {
 	diff := "+func Hello() string {\n+\treturn \"hello\"\n+}"
-	prompt := BuildUserPrompt("Greeting", "Add greeting func", diff, true, "")
+	prompt := BuildUserPrompt("Greeting", "Add greeting func", diff, "", true, "")
 
 	if !strings.Contains(prompt, "```diff") {
 		t.Error("expected diff fence")
@@ -75,3 +75,44 @@ func TestBuildUserPrompt_DiffIncluded(t *testing.T) {
 		t.Error("expected diff content in prompt")
 	}
 }
+
+func TestBuildSystemPrompt_WithPatterns(t *testing.T) {
+	patterns := "## Naming: use snake_case for functions"
+	prompt := BuildSystemPrompt("", patterns)
+	if !strings.Contains(prompt, "Language Patterns") {
+		t.Error("expected patterns section header")
+	}
+	if !strings.Contains(prompt, "snake_case") {
+		t.Error("expected patterns content")
+	}
+}
+
+func TestBuildSystemPrompt_WithBoth(t *testing.T) {
+	conventions := "Run mix format before commit"
+	patterns := "Use pipe operator for transformations"
+	prompt := BuildSystemPrompt(conventions, patterns)
+	if !strings.Contains(prompt, "Repository Conventions") {
+		t.Error("expected conventions section")
+	}
+	if !strings.Contains(prompt, "Language Patterns") {
+		t.Error("expected patterns section")
+	}
+}
+
+func TestBuildUserPrompt_WithFileContext(t *testing.T) {
+	fileContext := "--- main.go ---\npackage main\n"
+	prompt := BuildUserPrompt("Fix", "desc", "diff here", fileContext, true, "")
+	if !strings.Contains(prompt, "Full File Context") {
+		t.Error("expected file context section")
+	}
+	if !strings.Contains(prompt, "package main") {
+		t.Error("expected file content in prompt")
+	}
+}
+
+func TestBuildUserPrompt_WithoutFileContext(t *testing.T) {
+	prompt := BuildUserPrompt("Fix", "desc", "diff here", "", true, "")
+	if strings.Contains(prompt, "Full File Context") {
+		t.Error("should not include file context section when empty")
+	}
+}