chore: delete unused files across opencode

Remove dead scripts, spec files, TUI components, and a duplicate test
fixture that knip flagged as unused. Verified each by grepping for any
dynamic / string / URL references before deleting.

Files removed:
- script/check-migrations.ts (no references)
- script/time.ts (no references)
- script/trace-imports.ts (no references; hardcoded dev path)
- specs/v2/api.ts (@ts-nocheck design spec, no references)
- src/cli/cmd/tui/component/dialog-tag.tsx (not imported)
- src/cli/cmd/tui/routes/session/dialog-subagent.tsx (not imported)
- src/cli/cmd/tui/routes/session/footer.tsx (session uses subagent-footer)
- test/fixture/flock-worker.ts (duplicate; core has its own copy)