Map task lifecycle events to adapter DTOs

[cafitac]

Summary

• add pure TaskEvent mapping helpers for Gateway SSE event payloads, existing channel actions, and Gateway/MCP task status payloads
• export the helpers from hermit_agent.orchestrators for later adapter-boundary extraction
• update refactor/readiness docs and changelog to record that this is a behavior-preserving mapping-only slice

Behavior

• No CLI, MCP, Gateway, or channel delivery path is rewired in this PR.
• Unknown/non-lifecycle SSE events return None rather than inventing a misleading lifecycle state.

Tests

• RED observed first: .venv/bin/python -m pytest tests/test_orchestrator_event_mapping.py -q failed because the new helpers were not exported yet
• .venv/bin/python -m pytest tests/test_orchestrator_event_mapping.py -q -> 5 passed
• .venv/bin/python -m pytest tests/test_orchestrator_event_mapping.py tests/test_orchestrator_contracts.py tests/test_orchestrator_prompt_mapping.py tests/test_mcp_notification_wire.py tests/test_mcp_channel_buffering.py -q -> 45 passed
• .venv/bin/python -m pytest tests/ -q -> 1071 passed