phase5-plan: continuous enforcement hardening plan

[rafael5]

Summary

Phase 5 first move per AI-discoverability-plan.md §6 + AI-discoverability-architecture.md §5 "Operational invariants". Authors the plan doc that the four enforcement gates will be built against. No enforcement gates ship in this PR — Tracks A–E land as their own PRs per the calling convention.

Phase 5 scope

Four enforcement gates, each watching one architectural promise that's currently held by maintainer discipline:

Gate	Watches	Catches
Freshness	verified_on across repo.meta.json + recipes	Stale claims drifting under our feet
Link-check	Every URL in llms.txt + every *_url in tools.json + doc in task_index.json	Pointer-URL rot (kills "pointers, not facts")
License-reconcile	Declared license vs LICENSE-file signature	Silent license drift inside the org
Schema-version	schema_compat bumps on *.schema.json	A consumer pinned to v1 breaking when org publishes v2

Track layout

Five tracks. A–D are independent (no file overlap, no shared mutable state); any pair is safe to run in parallel. Each track ships:

• one new profile/build/check-<gate>.py (TDD-covered)
• one new Make target (make check-freshness / check-links / check-licenses / check-schema-compat)
• one new CI step

PR-mode runs the --offline variants (fast, no flake). Weekly cron runs the live-fetch variants on the same firing as the existing handshake-live cron.

Track B carries a Phase 4 follow-up as B0: fix validate-repo-meta.py's UTF-8-decode bug on binary exposes.* URLs (the workaround ARGS=--no-resolve exists today; B0 removes the need).

Phase 5 done definition

Seven criteria (plan §10). When all green, the framework's operational loop is complete — every architectural promise has an automated watchdog.

Other changes

• docs/ai-discoverability/README.md — Phase 5 row added to the phase table.
• docs/ai-discoverability/AI-discoverability-architecture.md — "Phase 5 outlook" section rewritten to point at the plan + summarise the five-track shape.

Test plan

• make check-docs-prose — clean
• make recipes-check — 4/4 clean
• make handshake — 8/8 steps green
• CI green