feat: add self-verification gate before generator marks story done

Generator must now verify each acceptance criterion against actual
code before setting passes: true. Acts as a first filter before
the evaluator runs, reducing false completions.

prompts/generator/_base.md

@@ -19,7 +19,11 @@ Do NOT start implementation until steps 1-5 are complete.
 - **Run quality gates** before committing. Check for common tools (`npm test`, `pytest`, `cargo test`, `make test`, `go test ./...`) and run what's available. If no test tooling exists, verify manually.
 - **Commit** with message: `feat: [Story ID] - [Story Title]`
 
-## After Completing
+## Before Marking Done
+
+Go through each acceptance criterion in the story and verify your work satisfies it. Check the actual code, not your memory of what you wrote. If any criterion is not met, fix it before continuing. Do NOT set `passes: true` until every criterion is verified.
+
+## After Verified
 
 1. Update `.loop/prd.json` — set `passes: true` for the story
 2. Append a summary to `.loop/progress.md` — what was done and which files were changed