[Tooling] knip hygiene: stale jsdom ignoreDependencies (post-#4435); prop-types ignore missing mode rationale (rode along in #4490)

[justin808]

Summary

Two knip-config hygiene items introduced by the fix wave. Stale ignore entries are how future dead code becomes invisible (the same class #4408 addressed).

Findings

1. Stale jsdom ignoreDependencies entry (post-#4435)

knip.ts:50's root jsdom ignore is now hint-flagged after #4435 removed the node-renderer jsdom@^16.5.0 devDep. Verify across the four invocation states per the #4425 policy (default, --production, and the CI passes), then delete. Also re-check whether the root jsdom@^22.1.0 devDep is still needed given jest-environment-jsdom.

2. prop-types ignore missing mode rationale (rode along in #4490)

The prop-types entry (~knip.ts L250) is legitimate — used by react_on_rails/spec/dummy/client/app/startup/HelloWorldES5.jsx:3 — but it landed in #4490 with no mention and lacks the sibling entries' mode-rationale comment, so it emits a permanent default-mode hint. Add the rationale comment (and confirm it belongs in the --production vs default set).

Acceptance criteria

• pnpm knip and pnpm knip --production emit no stale hint for jsdom or prop-types.
• Remaining ignores carry mode-rationale comments consistent with the Prune stale knip ignores and enforce binaries in CI (#4408) #4425 policy.

Audit ledger: #4010 (comment)

Post-merge audit follow-up (fix-wave a6c5867..ed5049f). Filed by Claude Code on behalf of Justin Gordon.