ci: gate PRs on local links; stop publishing the ADRs

[lesnik512]

Why

mkdocs build --strict validates links inside docs/ only. Root Markdown, .github/
and docs/agents/ are unchecked — AGENTS.md already says so. Nothing catches
a broken relative link in AGENTS.md, CONTEXT.md, or the README.

A relative link breaks because a diff broke it. Unlike an external URL it needs no network
and cannot flake, so it belongs on the PR that breaks it rather than in a scheduled job.

Design

One job in _checks.yml, so it runs wherever _checks.yml already runs. --offline blocks
all network requests and excludes external URLs rather than erroring on them, which is
what makes the gate deterministic and safe to block on. No exclusions, no token, no schedule.

The ADRs stop being published. not_in_nav existed for exactly one reason — build them
so --strict validates their outgoing links. The gate does that now, from disk, so /adr/
moves to exclude_docs.

Non-goals

• No external link checking here. That runs weekly in modern-python/.github and stays
  there; sharing it across repos was considered and rejected (Offer the link checker to the other repos as a reusable workflow .github#64).

• --include-fragments stays off — and this repo is the reason. Turning it on fails
  five links in docs/ as Cannot find fragment, and all five resolve correctly on the
  built site:

  Heading	python-markdown slug (what MkDocs emits)
  Slow handlers — dedicated queue	slow-handlers-dedicated-queue
  Drain & lifecycle	drain-lifecycle
  vs. FastStream + `KafkaBroker` / `RabbitBroker` directly	vs-faststream-kafkabroker-rabbitbroker-directly

  lychee computes GitHub-flavoured slugs, which leave a doubled hyphen where an em dash,
  & or backticks were. Verified by running markdown.extensions.toc.slugify directly.

Verification

• lychee --offline on this tree: 241 total, 202 OK, 0 errors — the gate lands green.
• just docs-build passes, and site/adr/ is confirmed absent from the build.
• No Python changed, so the test suite is untouched by this diff and I have not run it.

---

Before merging

• Adding a fact anywhere? No prose added; the check is executable.
• Rejected an alternative? The fragments finding is recorded in the commit message
  and Add the offline link gate to the 18 green repos, and unpublish the ADRs in the two that have them .github#66, where it applies to 18 repos rather than only this one.
• just lint-ci / just test — not run; this diff touches only CI config and
  mkdocs.yml.

[github-actions]

Benchmark gate

✅ gate passed

scenario	msg/s	delete/msg	WALrec/msg	WALB/msg	fpi	upd	del	dead_tup
consumer/w1/b10	1065	1.000	6.79	917	242	5000	5000	10000
consumer/w1/b100	1061	1.000	6.82	920	243	5000	5000	10000
consumer/w2/b10	1201	1.000	6.81	970	242	5000	5000	10000
consumer/w2/b100	1384	1.000	6.73	956	243	5000	5000	10000
consumer/w4/b10	1119	1.000	6.82	996	264	5000	5000	10000
consumer/w4/b100	1540	1.000	6.87	1039	245	5000	5000	10000
consumer/w1/b100/tfbs100	5860	0.010	6.07	1116	243	5000	5000	10000
producer/w1/b100	2393	0.000	3.04	584	0	0	0	0

Gated (fails the build): delete_calls + tuple counters (upd/del/ins) + the producer's insert_calls, exact; select_calls within +2; wal_records within a 10% band. msg/s, WAL bytes and total calls are informational (timing/FPI noise).