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