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.go

@@ -26,10 +26,11 @@ func NewClient(baseURL, token string) *Client {
 
 // PullRequest holds relevant PR metadata.
 type PullRequest struct {
-	Title  string `json:"title"`
-	Body   string `json:"body"`
-	Head   struct {
+	Title string `json:"title"`
+	Body  string `json:"body"`
+	Head  struct {
 		Sha string `json:"sha"`
+		Ref string `json:"ref"`
 	} `json:"head"`
 }
 
@@ -41,6 +42,12 @@ type CommitStatus struct {
 	TargetURL   string `json:"target_url"`
 }
 
+// ChangedFile represents a file modified in a PR.
+type ChangedFile struct {
+	Filename string `json:"filename"`
+	Status   string `json:"status"`
+}
+
 // GetPullRequest fetches PR metadata.
 func (c *Client) GetPullRequest(owner, repo string, number int) (*PullRequest, error) {
 	url := fmt.Sprintf("%s/api/v1/repos/%s/%s/pulls/%d", c.BaseURL, owner, repo, number)
@@ -65,6 +72,20 @@ func (c *Client) GetPullRequestDiff(owner, repo string, number int) (string, err
 	return string(body), nil
 }
 
+// GetPullRequestFiles fetches the list of files changed in a PR.
+func (c *Client) GetPullRequestFiles(owner, repo string, number int) ([]ChangedFile, error) {
+	url := fmt.Sprintf("%s/api/v1/repos/%s/%s/pulls/%d/files", c.BaseURL, owner, repo, number)
+	body, err := c.doGet(url)
+	if err != nil {
+		return nil, fmt.Errorf("fetch PR files: %w", err)
+	}
+	var files []ChangedFile
+	if err := json.Unmarshal(body, &files); err != nil {
+		return nil, fmt.Errorf("parse PR files JSON: %w", err)
+	}
+	return files, nil
+}
+
 // GetCommitStatuses fetches CI statuses for a commit SHA.
 func (c *Client) GetCommitStatuses(owner, repo, sha string) ([]CommitStatus, error) {
 	url := fmt.Sprintf("%s/api/v1/repos/%s/%s/commits/%s/statuses", c.BaseURL, owner, repo, sha)
@@ -89,6 +110,16 @@ func (c *Client) GetFileContent(owner, repo, filepath string) (string, error) {
 	return string(body), nil
 }
 
+// GetFileContentRef fetches a file from a specific ref (branch/tag/sha) in a repo.
+func (c *Client) GetFileContentRef(owner, repo, filepath, ref string) (string, error) {
+	url := fmt.Sprintf("%s/api/v1/repos/%s/%s/raw/%s?ref=%s", c.BaseURL, owner, repo, filepath, ref)
+	body, err := c.doGet(url)
+	if err != nil {
+		return "", fmt.Errorf("fetch file %s@%s: %w", filepath, ref, err)
+	}
+	return string(body), nil
+}
+
 // PostReview submits a review to a PR.
 // event should be "APPROVED" or "REQUEST_CHANGES".
 func (c *Client) PostReview(owner, repo string, number int, event, body string) error {