feat: authorize GitHub AI engagement

[DanNicolau]

Summary

• add state-specific, fail-closed GitHub actor authorization under workflow.engagement
• verify collaborator, organization, team, bot, and GitHub App identities for webhook and polling ingestion
• persist structured engagement decisions and durable generated-resource IDs
• protect built-in and plugin lifecycle starts and checkpoint resumes with the same policy gate
• incorporate the safe configuration and traceability ideas from feat: user gated tasks #8 while retaining ID-based recursion prevention

Tests

• cargo fmt --all -- --check
• cargo check --workspace
• cargo test --workspace
• cd web && npm run build
• git diff --check

Closes #7

Supersedes #8.