docs(skill): reduce file-mailbox coordination round trips

[fglock]

Summary

• add bounded execution envelopes to reduce worker approval round trips
• document self-advancing dependency handoffs and explicit stop conditions
• replace coordinator-granted local build slots with decentralized admission
  under a published concurrency limit
• make long corpus/build gates observable with GATE_STARTED/GATE_FINISHED,
  resumable session, log, and ETA metadata
• distinguish queued preparation from active implementation by requiring an
  activity kind, exact worktree/branch/base, and first edited file or gate
  process evidence
• require source repository + ref + exact commit for cross-checkout handoffs so
  workers do not wait on SHAs absent from their object database
• audit observable state transitions rather than treating mailbox volume or an
  “ACTIVE” label as implementation progress

Validation

• warning-free full make at 684a3e65b: exit 0, all 17 tasks in 5m53s
• Joni packaging verification, all Joni tests, and five unit shards passed
• skill frontmatter, required keys, 419-line size, and git diff --check pass
• the supplied quick_validate.py could not start because this environment
  lacks its PyYAML dependency; no dependency was installed during active builds
• protocol exercised by Phase 36: observable worktree checks exposed a missing
  DEFINE start, exact cross-repository fetch instructions unblocked workers,
  and conditional implementation queues removed coordinator round trips

Stacking

This is based on chore/perl5-sync-35694276 and should be retargeted to
master after that PR merges.