Generate contextual titles for explicit implement issues

[sepo-agent-app]

Summary

• Add a metadata-only prompt for explicit /implement requests on PRs/discussions.
• Run that prompt best-effort before creating the tracking issue, while preserving the explicit route and no-approval dispatch path.
• Merge generated issue title/body only when the response route matches, with deterministic fallback behavior.
• Document the behavior and add regression coverage.

Tests

• npm --prefix .agent run build
• node --test .agent/dist/__tests__/resolve-dispatch-cli.test.js .agent/dist/__tests__/triage.test.js .agent/dist/__tests__/envelope.test.js
• git diff --check

Closes #234