feat(atlas): support plugin tools with metadata.sessionId in orchestration hook

The tool-execute-after hook only processed the built-in 'task' tool.
Plugin tools (e.g. custom-agent-bridge delegate) that set
context.metadata({ metadata: { sessionId } }) were ignored by atlas
orchestration (boulder tracking, verification reminders).

- Add extractSessionIdFromMetadata() to read sessionId from tool
  metadata, preferred over text-based extraction
- Extend tool-execute-after guard to also process any tool that has
  metadata.sessionId set (not just tool === 'task')
- Recognize 'Background delegate launched' as a background launch
- Add tests for metadata-based session ID extraction