fix(changelog): v0.18.0 notes hold Highlights and What's Changed only

[chaliy]

What changed

v0.18.0 release notes violated the documented convention (knowledge/operations/release-process.md: Highlights + What's Changed, no Added/Fixed/Changed):

• stale lead highlight perf(builtins): cache pre-built clap Commands for ported coreutils builtins #2347 shipped in v0.17.1, not in this release
• verbatim C ABI bullet in Highlights and Added
• every curated PR (e.g. fix(host-call): enforce the execution deadline while parked #2349) listed twice: curated section + What's Changed
• ten curated bullets with no PR link (the chore(maintenance): refresh dependencies and resolve security findings #2378 maintenance scope)

Now the section holds 2 Highlights bullets + the full What's Changed list. Adds scripts/check_changelog.py (R0 well-formed links, R1 highlights carry links, R2 no dupes incl. containment, R3 no stale refs from older releases, R4 Highlights/What's Changed only; --scaffold builds What's Changed from git log merge order), wired into just check, 9 unit tests, release-process documents the rules.

Before / After

Before: checker reports 12 violations on the old section (10 orphans + 1 dupe + 1 stale, plus 3 unexpected subsections).
After: just check-changelog exits 0; python3 -m unittest discover -s scripts/tests passes for the new tests (one pre-existing wasm_bindgen_version failure, fails on clean main too).

Risk

• Low. CHANGELOG text + lint script only; no runtime code.
• The published GitHub Release body still shows the old notes and needs a follow-up gh release edit after merge.

Checklist

• Tests added or updated
• Backward compatibility considered (docs-only)

Produced by yolop

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