fix(ast-grep): fix ast_grep_replace silent write failure

[jsl9208]

Summary

• ast_grep_replace with dryRun=false silently fails to write files because --json=compact and --update-all are passed in the same CLI invocation, and ast-grep ignores --update-all when --json mode is active
• The tool reports "N replacement(s)" identically for both success and failure, causing AI agents to assume files were modified and enter incorrect debugging paths

Changes

• Split runSg() into two separate CLI invocations when both rewrite and updateAll are requested:
  1. --json=compact (without --update-all) to collect match results
  2. --update-all (without --json) to perform actual file writes
• Preserve match report on write failure ({ ...jsonResult, error })

Testing

Verified with real ast-grep binary (v0.40.5):

1. dryRun=true — reports matches, file unchanged ✅
2. dryRun=false — reports matches, file modified ✅

PR Checklist

• Code follows project conventions
• bun run typecheck passes
• bun run build succeeds
• Tested locally with OpenCode
• Updated documentation if needed (README, AGENTS.md)
• No version changes in package.json

Related Issues

None found — this bug was not previously reported.

---

Summary by cubic

Fixes ast_grep_replace writing no changes when using rewrite+updateAll by separating read and write passes so file updates actually happen. Also preserves match results and surfaces write errors instead of reporting false success.

• Bug Fixes
  • Run ast-grep twice when rewrite and updateAll are both set: first with --json=compact, then with --update-all.
  • Return match report with an error message if the write pass fails.
  • Verified with ast-grep v0.40.5: dryRun=true reports matches without changes; dryRun=false updates files.

^{Written for commit fec12b6. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 1 file

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Requires human review: The separate write pass logic introduces new async subprocess handling with timeout and error paths. While the fix addresses a real CLI quirk, the new code path (lines 163-179) hasn't been validated…}