feat: harden v1.2 mesh bootstrap

[nillo]

Summary

Ships the v1.2 mesh bootstrap hardening slice. This adds explicit worker startup prompts, state-scoped room/session registries for port-base and named-runtime flows, stricter daemon identity matching, safer Claude channel argv shape, and read-only mesh-lane affordances.

Changes

• Add explicit mesh worker startup prompts that survive the instance-env shell wrapper.
• Scope room/session registries to the active state dir for daemon, session, and kill paths.
• Tighten live daemon reuse around instance id, daemon identity, pid, control port, project root, and state dir.
• Switch Claude channel injection to --channels=<value> / --dangerously-load-development-channels=<value>.
• Default mesh Claude launches to approved-channel loading under CONTEXTRELAY_ENABLE_MESH=1.
• Auto-enable named sessions for read-only mesh lanes and keep write-capable worktree conflict checks intact.
• Bump release surface to 1.2.0 and document the slice in CHANGELOG.md.

Testing

• bun run check passed after the version/changelog update:
  • 690 src tests passed
  • 3 example tests passed
  • CLI/plugin/docs sync checks passed
  • plugin validation, plugin version alignment, and lockfile checks passed
• Registry scoping audit: no non-test production callers of unscoped registry helpers outside src/session/registry.ts; daemon/session/kill paths use state-scoped registry paths.
• Prior tmux final smoke produced ready notes from default and named workers.

Follow-ups

• Cleanup migration for orphan project-root rooms.json / sessions.json.
• Claude allowlist UX line polish.
• Regression coverage for reused/orphaned-TUI restart path.
• Permission-flip-aware shared-worktree re-check if lane permissions become mutable in-place.

Notes

The untracked game/ folder from the separate mini-game task is intentionally not included in this PR.