feat: sentinel-based review cleanup + system prompt file + security review

Sentinel-based cleanup:
- Reviews embed <!-- review-bot:NAME --> in body (hidden HTML comment)
- Cleanup matches by sentinel, not token identity
- Each reviewer-name is a logical identity (sonnet, gpt, security)
- Same token can run multiple review types without conflict
- No extra API scopes needed

System prompt file (--system-prompt-file / SYSTEM_PROMPT_FILE):
- Loads a local file with additional review instructions
- Appended to system base as "Additional Review Instructions"
- Enables specialized reviews (security, performance, etc.)
- Partially addresses #5

Security review:
- SECURITY_REVIEW.md prompt focused on vulnerabilities
- 3rd CI matrix entry using same token, different prompt
- Focus: injection, auth, secrets, input validation, crypto, races

CI changes:
- REVIEWER_NAME passed from matrix.name
- SYSTEM_PROMPT_FILE passed from matrix (empty for standard reviews)
- 3 reviewers: sonnet (general), gpt (general), security (focused)

gitea/client.go

@@ -279,6 +279,7 @@ func (c *Client) GetAllFilesInPath(ctx context.Context, owner, repo, path string
 // Review represents a pull request review from the Gitea API.
 type Review struct {
 	ID   int64  `json:"id"`
+	Body string `json:"body"`
 	User struct {
 		Login string `json:"login"`
 	} `json:"user"`