feat(ci): semantic package-lock.json diff as an upserted PR comment (#65206)

git diff on a lockfile is unreadable: npm reorders entries, rewrites
integrity hashes, and moves packages between nesting levels, so a
one-line package.json bump produces a thousand-line textual diff.

scripts/ci/lockfile_diff.py instead parses the `packages` map out of
both versions of every tracked package-lock.json (via `git show`),
reduces each to {install path: version}, and set-diffs the maps —
reorder/hash churn vanishes, leaving only actual version movement
(added / removed / updated, with nested dedup copies tracked
separately).

The lockfile-diff workflow posts the result as a Markdown table in a
PR comment gated behind a hidden marker: subsequent pushes PATCH the
existing comment instead of stacking new ones, and a push that reverts
all lockfile changes updates the comment to say so. Advisory only —
never fails on findings; fork PRs (read-only token) degrade to a
warning.

Wired through the ci.yml orchestrator with a new npm_lock lane in
classify_changes.py (fails open on .github/ changes per the existing
contract).

.github/actions/detect-changes/action.yml

@@ -24,6 +24,9 @@ outputs:
   deps:
     description: Check pyproject.toml dependency upper bounds.
     value: ${{ steps.classify.outputs.deps }}
+  npm_lock:
+    description: Post/update the semantic package-lock.json diff PR comment.
+    value: ${{ steps.classify.outputs.npm_lock }}
   mcp_catalog:
     description: Require MCP catalog security review label.
     value: ${{ steps.classify.outputs.mcp_catalog }}