Summary

• add first-class fork-mode watch/session state and deterministic fork discovery/remote setup
• reuse fork branches in worktrees while keeping upstream as the issue and PR source of truth
• teach prompt/session/cleanup flows about fork push targets and cross-repo PR heads
• document fork-mode usage in the README and contributor guide

Benefits

• supports repositories where Vigilante should push through a bot or personal fork instead of upstream
• keeps resume, redispatch, cleanup, and PR maintenance aligned with the actual push remote
• gives coding agents explicit fork-aware PR instructions without replacing the existing repo-specific skills
• makes the new operator workflow visible in both quickstart and contributor-facing docs

Validation

• go test ./...

Closes #340