chore: establish incremental TS migration scaffolding and CI gate

[birme]

Summary

• Implements chore: establish incremental TS migration scaffolding and CI gate #372: groundwork so engine/'s remaining JS modules can be converted to TypeScript one file at a time without breaking master.

Changes

• tsconfig.json: keep allowJs: true (mixed JS/TS tree already supported), add forceConsistentCasingInFileNames: true, and a comment recording that checkJs/strict are deliberately deferred (they land per-file, and globally only in the final slice chore: migrate jasmine spec suite to TypeScript and tighten compiler strictness #376). No global checkJs/strict — that would type-check 7,500 LOC of untyped JS and break the build.
• CONTRIBUTING.md: new "TypeScript migration" section — target strictness level, per-file conversion checklist (git mv .js→.ts, type exports, preserve public API/runtime behavior, build+test green, one module per commit), and the note that global strict flags land last in chore: migrate jasmine spec suite to TypeScript and tighten compiler strictness #376.
• CI: no new workflow — .github/workflows/nodejs.yml already runs npm run build (tsc) on push, which is the type-check gate.

Test plan

• PROOF: npm run build && npm test → 113 specs, 0 failures, 7 pending (identical to baseline; no behavior change).

Closes #372

🤖 Automated via Channel Engine Dev daily-backlog-pr skill

[birme]

pr-reviewer verdict: APPROVE

PROOF: npm run build && npm test → 113 specs, 0 failures, 7 pending (identical to master baseline e9d9a5e — no behavior change).

• tsconfig: allowJs stays on (mixed tree still compiles), only functional add is forceConsistentCasingInFileNames: true (safe hygiene, surfaced no errors). checkJs/strict correctly NOT enabled globally — the issue's "or equivalent" wording + documented deferral to per-file migration and #376 is a sound reading; enabling them now would break the build and violate the "builds cleanly / no behavior change" acceptance criteria.
• CONTRIBUTING.md: contains (a) target strictness, (b) per-file conversion checklist, (c) note that global strict flags land in #376 — all three acceptance criteria met.
• No trademarks. Commit message format correct.

Merging.