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.

gitea/client_test.go

@@ -193,3 +193,48 @@ func TestGetFileContent(t *testing.T) {
 		t.Errorf("expected %q, got %q", expected, got)
 	}
 }
+
+func TestGetPullRequestFiles(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path != "/api/v1/repos/owner/repo/pulls/1/files" {
+			t.Errorf("unexpected path: %s", r.URL.Path)
+		}
+		w.Header().Set("Content-Type", "application/json")
+		w.Write([]byte(`[{"filename":"main.go","status":"modified"},{"filename":"old.go","status":"removed"}]`))
+	}))
+	defer server.Close()
+
+	client := NewClient(server.URL, "test-token")
+	files, err := client.GetPullRequestFiles("owner", "repo", 1)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if len(files) != 2 {
+		t.Fatalf("expected 2 files, got %d", len(files))
+	}
+	if files[0].Filename != "main.go" || files[0].Status != "modified" {
+		t.Errorf("unexpected first file: %+v", files[0])
+	}
+}
+
+func TestGetFileContentRef(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path != "/api/v1/repos/owner/repo/raw/main.go" {
+			t.Errorf("unexpected path: %s", r.URL.Path)
+		}
+		if r.URL.Query().Get("ref") != "feature-branch" {
+			t.Errorf("unexpected ref: %s", r.URL.Query().Get("ref"))
+		}
+		w.Write([]byte("package main\n"))
+	}))
+	defer server.Close()
+
+	client := NewClient(server.URL, "test-token")
+	content, err := client.GetFileContentRef("owner", "repo", "main.go", "feature-branch")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if content != "package main\n" {
+		t.Errorf("unexpected content: %q", content)
+	}
+}