fix(repo): remove stray root files and resolve duplicate unreleased changelog heading (closes #908)

[mopxam-art]

Overview

Closes #908 by removing stray debug and working files from the repository root, ensuring tmp-* patterns are gitignored alongside .dockerignore, and resolving the duplicate ## [Unreleased] heading inside CHANGELOG.md.

Changes

• Removed stray files from repository root:
  • tmp-idempotency-check.js (debug script)
  • FIX_SUMMARY.md
  • IMPLEMENTATION_CHECKLIST.md
• .gitignore:
  • Added tmp-* pattern to match existing .dockerignore rule and prevent ephemeral local debug artifacts from being staged.
• CHANGELOG.md:
  • Wrapped copyable template heading into comment <!-- ## [Unreleased] --> so automated release tools and search indices only find the single canonical top-level ## [Unreleased] section.
• src/__tests__/repoHygiene.test.ts:
  • Added unit tests covering repository structure, absence of prohibited root files, gitignore glob patterns, and changelog heading formatting.

Verification

• All unit tests pass cleanly (npx jest src/__tests__/repoHygiene.test.ts --forceExit).
• TypeScript typecheck passes with 0 errors (npx tsc --noEmit).
• ESLint passes with 0 warnings/errors (npx eslint src/__tests__/repoHygiene.test.ts).
• Production build compiles cleanly (npm run build).