Update docs: AGENTS.md matches the checks that exist

[TheGreatAxios]

Fixes CL-7126 — https://linear.app/abklabs/issue/CL-7126
Fixes CL-7142 — https://linear.app/abklabs/issue/CL-7142

Problem

• AGENTS.md's "Working conventions" (line 60-61 before this change) states
  "Merged line coverage floor: 80%." Nothing in the repo measures or
  enforces it: no bunfig.toml coverage config, no --coverage flag in
  any script, no CI step. The same sentence says "no coverage theater" —
  the floor sentence contradicts it.
• AGENTS.md's "Conventions a check enforces" (the check:tool-package-pins
  bullet) says a package's src/ changing without a version bump is
  "currently unchecked; ticket before relying on it." But
  package.json's check:structural already runs
  check:tool-package-freshness (scripts/checks/tool-package-freshness.ts),
  which does exactly that: diffs a tool package's src/ against a base ref
  and flags a change with no version bump.

Change

• Remove the coverage-floor sentence; keep the "meaningful red/green tests
  only" guidance intact. No coverage tooling added.
• Rewrite the tool-package-pins bullet to name check:tool-package-freshness
  as the enforcing mechanism: it diffs against a base ref (CI passes
  CHECK_BASE_REF; locally it falls back to the merge base with
  origin/main), and no-ops with no base ref available, deferring to CI
  as the authoritative run.

Tests

Doc-only change — no code touched. Verified both premises against source
before editing (grepped for coverage config/CI steps; read
scripts/checks/tool-package-freshness.ts and confirmed it runs under
check:structural in package.json). Ran bunx prettier --check AGENTS.md
and the full bun run check gate; typecheck, lint, and check:structural
are clean. Two unrelated test failures surfaced during parallel runs
(@corbits/agent-lifecycle, @corbits/settings-ui) but both pass in
isolation — timing flakes under heavy concurrent load on this run, not
caused by this change.