feat: wire IM channel runtime

[warren618]

Summary

• add shared IM channel discovery, dependency-gated status, manager, message bus, pairing store, and SessionService runtime wiring
• expose channel controls through REST, CLI, and Settings UI with i18n coverage
• declare channel dependency extras and add focused backend/frontend regression tests
• remove legacy-name references from the committed tree

Verification

• PYTHONPATH=agent pytest agent/tests/test_channels_runtime.py agent/tests/test_channels_api.py agent/tests/test_cli_channels.py agent/tests/test_packaging_dependencies.py -q
• python -m compileall -q agent/src/channels agent/src/channelsui agent/src/security/workspace_access.py agent/src/security/network.py agent/src/security/workspace_policy.py agent/src/session/goal_state.py agent/src/session/webui_turns.py agent/src/utils/media_decode.py agent/api_server.py agent/cli/_legacy.py
• cd frontend && npm test -- SettingsChannels --run && npm test -- i18n --run
• cd frontend && npm run build
• final committed tree legacy-name scan returned no matches