chore: remove dead test and script files

Dead-code sweep via knip. All files below have zero importers and were
leftovers from the local-echo overlay exploration or duplicated by files
under test/mocks/.

Deleted:
- test/respawn-test-utils.ts (728-line duplicate of test/mocks/*)
- test/input-echo-test.mjs
- test/local-echo-*.mjs (7 files)
- test/manual/*.mjs (10 files; dir removed)
- scripts/remotion/components/TerminalScreen.tsx (unused Remotion demo)

Also cleaned stale JSDoc references to the removed
respawn-test-utils.ts in test/mocks/mock-session.ts and
test/mocks/test-helpers.ts.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>