fix: address review findings (path restriction, login cross-check, README)

- system-prompt-file: reject absolute paths and paths containing ".."
  Prevents reading arbitrary files outside the workspace on shared runners.
- Cleanup: cross-check r.User.Login == posted.User.Login before deletion
  Defense-in-depth: only attempt to delete reviews from same author.
  Flagged by both sonnet and security reviewers.
- README: fix wording (cleanup happens after posting, not before)

Issues filed for deferred work:
- #24: Consistent url.PathEscape across all client endpoints
- #25: Binary signature verification for supply-chain hardening

README.md

@@ -7,7 +7,7 @@ AI-powered code review bot for Gitea pull requests. Fetches diff + context, send
 - **Multi-provider**: OpenAI-compatible and Anthropic Messages API
 - **Context-aware**: Fetches full file content, conventions, language patterns, CI status
 - **Smart budget**: Automatically trims context to fit model token limits
-- **Idempotent reviews**: Deletes previous review before posting new one (one review per bot)
+- **Idempotent reviews**: Posts new review, then cleans up stale ones (one review per bot)
 - **Custom prompts**: Load additional instructions from a file (e.g. security-focused review)
 - **Zero dependencies**: Go stdlib only
 

cmd/review-bot/main.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"log"
 	"os"
+	"path/filepath"
 	"strconv"
 	"strings"
 	"time"
@@ -154,6 +155,12 @@ func main() {
 	// Step 6b: Load additional system prompt if specified
 	additionalPrompt := ""
 	if *systemPromptFile != "" {
+		if filepath.IsAbs(*systemPromptFile) {
+			log.Fatalf("system-prompt-file must be a relative path (got %q)", *systemPromptFile)
+		}
+		if strings.Contains(*systemPromptFile, "..") {
+			log.Fatalf("system-prompt-file must not contain '..' (got %q)", *systemPromptFile)
+		}
 		data, err := os.ReadFile(*systemPromptFile)
 		if err != nil {
 			log.Fatalf("Failed to read system prompt file %q: %v", *systemPromptFile, err)
@@ -227,7 +234,7 @@ func main() {
 			log.Printf("Warning: could not list existing reviews: %v", err)
 		} else {
 			for _, r := range reviews {
-				if r.ID != posted.ID && strings.Contains(r.Body, sentinel) {
+				if r.ID != posted.ID && r.User.Login == posted.User.Login && strings.Contains(r.Body, sentinel) {
 					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 {