fix(health): exempt retired claims from the lint stale check

[luciferlive112116]

what changed

health.lint (vouch lint) ran its stale-claim check over every claim
with no status guard. it now exempts retired statuses
(superseded / archived / redacted) from that check, matching
vouch metrics and vouch digest.

fixes #478.

why

lint is documented as the "user-actionable problems" subset of the health
checks. a retired claim is terminal and is not expected to be refreshed, so
warning that it is stale_claim past the freshness window is pure noise.

the two sibling surfaces that compute the same "stale" notion already exempt
retired claims, and their comments name lint as the behaviour they are
matching:

• metrics.py: "matching lint, which only warns on claims a reader would
  still trust" / "retired claims are intentionally exempt — they're not
  expected to be refreshed."
• digest.py: "same freshness anchor and thresholds as vouch metrics /
  vouch lint."

lint was the odd one out; this brings it in line.

what might break

nothing on disk. no file moves, no field changes, no kb.* shape change.
vouch lint / vouch doctor simply stop emitting stale_claim findings for
already-retired claims — active claims are still flagged exactly as before, so
report.ok is unchanged (stale was a warning, never an error). the only
observable difference is fewer non-actionable warnings.

tests

• tests/test_health.py::test_lint_exempts_retired_claims_from_stale — a
  retired claim past the window is not flagged while an active one still is.
  fails on the previous code, passes now.
• ran locally: pytest tests/test_health.py tests/test_metrics.py tests/test_digest.py, ruff check src tests, mypy src
  (health.py clean).

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