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>

test/mocks/mock-session.ts

@@ -1,7 +1,5 @@
 /**
  * Shared MockSession for tests that need terminal simulation.
- *
- * Copied from test/respawn-test-utils.ts (the canonical, most complete version).
  * Used by respawn, route, and subagent tests.
  */
 import { EventEmitter } from 'node:events';