feat: delete previous review before posting new one (#6)

Before posting a review, the bot now:
1. Calls GET /api/v1/user to identify its own login
2. Lists all reviews on the PR
3. Deletes any existing reviews from itself
4. Posts the fresh review

This keeps PR threads clean — one review per bot at any time.

New Gitea client methods:
- GetAuthenticatedUser() — token self-identification
- ListReviews() — fetch reviews on a PR
- DeleteReview() — delete a review by ID

Flag: --update-existing / UPDATE_EXISTING (default true)
Set to false to preserve old behavior (stack reviews).

All delete failures are non-fatal (logged as warnings).

Closes #6

cmd/review-bot/main.go

@@ -33,6 +33,7 @@ func main() {
 	patternsRepo := flag.String("patterns-repo", envOrDefault("PATTERNS_REPO", ""), "Repo with language patterns (e.g. rodin/elixir-patterns)")
 	patternsFiles := flag.String("patterns-files", envOrDefault("PATTERNS_FILES", "README.md"), "Comma-separated file paths to fetch from patterns repo")
 	dryRun := flag.Bool("dry-run", false, "Print review to stdout instead of posting")
+	updateExisting := flag.Bool("update-existing", envOrDefaultBool("UPDATE_EXISTING", true), "Delete previous review from same bot before posting (default true)")
 	llmTemp := flag.Float64("llm-temperature", envOrDefaultFloat("LLM_TEMPERATURE", 0), "LLM temperature (0 = server default)")
 	llmTimeout := flag.Int("llm-timeout", envOrDefaultInt("LLM_TIMEOUT", 300), "LLM request timeout in seconds (default 300)")
 	llmProvider := flag.String("llm-provider", envOrDefault("LLM_PROVIDER", "openai"), "LLM API provider: openai or anthropic")
@@ -195,6 +196,29 @@ func main() {
 		return
 	}
 
+	// Delete previous reviews from this bot if update-existing is enabled
+	if *updateExisting {
+		login, err := giteaClient.GetAuthenticatedUser(ctx)
+		if err != nil {
+			log.Printf("Warning: could not determine reviewer identity: %v", err)
+		} else {
+			reviews, err := giteaClient.ListReviews(ctx, owner, repoName, prNumber)
+			if err != nil {
+				log.Printf("Warning: could not list existing reviews: %v", err)
+			} else {
+				for _, r := range reviews {
+					if r.User.Login == login {
+						if err := giteaClient.DeleteReview(ctx, owner, repoName, prNumber, r.ID); err != nil {
+							log.Printf("Warning: could not delete old review %d: %v", r.ID, err)
+						} else {
+							log.Printf("Deleted previous review %d", r.ID)
+						}
+					}
+				}
+			}
+		}
+	}
+
 	log.Printf("Posting review (event=%s)...", event)
 	if err := giteaClient.PostReview(ctx, owner, repoName, prNumber, event, reviewBody); err != nil {
 		log.Fatalf("Failed to post review: %v", err)
@@ -333,3 +357,11 @@ func envOrDefaultInt(key string, defaultVal int) int {
 	}
 	return defaultVal
 }
+
+func envOrDefaultBool(key string, defaultVal bool) bool {
+	v := os.Getenv(key)
+	if v == "" {
+		return defaultVal
+	}
+	return v == "true" || v == "1" || v == "yes"
+}