[codex] Add TUI plugin extensibility surface

[windoliver]

Summary

• add typed TUI extension registry contracts for plugin actions and operator panels
• project extension actions into the command palette and execute them through a plugin action context
• render trusted local extension panels in operator panel slots with docs/spec coverage
• add tmux E2E coverage for the extension panel/action path and harden the live Codex -> Claude handoff harness with isolated ports

Validation

• bun run tests/tui/codex-claude-handoff-e2e.ts --timeout 210000
• bun ./tests/e2e/tui-extension-review-loop.tmux.ts --timeout 60000
• bun run typecheck
• bun run build
• bun run check
• git diff --check