docs(changelog): record the lint retired-stale fix

[luciferlive112116]

what changed

adds the ## [Unreleased] → ### Fixed changelog entry for the lint
retired-stale fix that landed in #484 (issue #478). changelog only — no
code change.

this pr originally carried the fix itself; #484 landed the same change
first, so it has been reduced to the one piece that wasn't merged with it.

why

CONTRIBUTING requires a CHANGELOG.md entry under ## [Unreleased] for
user-visible changes, and #484 shipped health.py + tests without one.
lint no longer emitting stale_claim warnings for retired claims is
user-visible — anyone parsing vouch lint output sees a different set of
findings. ### Fixed is what the next release notes are rolled from
(CONTRIBUTING §"Release checklist" step 2), so without this the change
ships undocumented.

what might break

nothing. documentation only — no code, no on-disk shape, no kb.*
behaviour.

tests

• n/a — changelog only. no code paths touched.
• the behaviour itself is already covered by the tests that landed with
  fix(lint): exempt retired claims from stale check #484 (test_lint_exempts_retired_claims_from_stale_check,
  test_lint_flags_stale_active_claims).

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[luciferlive112116]

#484 landed the same fix first, so i've rebased this onto current test
and dropped my duplicate health.py + test changes — they're already in.

what's left is just the changelog entry, which #484 didn't include. the
retired-stale exemption is user-visible (vouch lint stops emitting
stale_claim for retired claims) and ### Fixed is what the next release
notes get rolled from, so it seemed worth not losing. happy to close if
you'd rather fold it in elsewhere.