fix(release): check-changelog-placement's docstring describes the retired count rule, and its allowlist loses multiplicity

Summary

Follow-up to #454 (deferred advisories from that PR's review round; the guard itself ships working).

Two loose ends in skills/release/check-changelog-placement.py:

1. Stale contract prose. The module docstring (lines ~15-27) and the final remediation diagnostic (~line 146) still describe the retired NET COUNT rule — "a branch must not increase the number of entry blocks parked", "Exit 0 when the count did not rise", "does not raise the count". The implemented rule is block-content identity (newly_parked), which never computes a count. A reader scripting against the documented contract gets the wrong predicate. The test comment at skills/release/tests/test_check_changelog_placement.py:164 carries the same stale rationale.

2. set() drops multiplicity. newly_parked builds known = set(blocks(original)), so a branch that adds a SECOND identical entry block under a published heading passes — both parked copies are members of the one-element set. Compare occurrences as a multiset (consume each base occurrence before classifying the parked remainder), which keeps the archive-repair case passing while catching a genuinely added duplicate.

Test plan

• Docstring, --help text and the failure diagnostic all describe the identity predicate
• Regression: base has one parked block, branch has two identical copies of it -> exit 1
• Existing archive-repair case (move an entry between headings) still exits 0

Start with skills/release/check-changelog-placement.py, its module docstring, --help text, and final diagnostic, then review the related cases in skills/release/tests/test_check_changelog_placement.py around line 164. Run the focused tests and add coverage for a duplicated parked block while preserving the existing archive-repair behavior; done means all documented messages describe the identity predicate and the test plan passes.