[CYPACK-726] Extract IActivitySink interface and LinearActivitySink implementation

[cyrusagent]

Summary

This PR implements Phase 2 of the activity sink extraction, creating the IActivitySink interface and LinearActivitySink implementation to decouple activity posting from IIssueTrackerService. This enables multiple activity sinks (Linear workspaces, GitHub, etc.) to receive session activities based on session context.

Changes

New Files

• packages/edge-worker/src/sinks/IActivitySink.ts - Interface defining activity sink contract
• packages/edge-worker/src/sinks/LinearActivitySink.ts - Linear-specific implementation wrapping IIssueTrackerService
• packages/edge-worker/src/sinks/index.ts - Barrel export for sinks
• packages/edge-worker/test/LinearActivitySink.test.ts - Comprehensive unit tests (20 tests)

Modified Files

• packages/edge-worker/src/index.ts - Exported IActivitySink and LinearActivitySink
• CHANGELOG.internal.md - Added entry documenting the changes

Implementation Details

IActivitySink Interface:

• readonly id: string - Unique identifier for the sink (workspace ID)
• postActivity(sessionId, activity) - Post activities to agent sessions
• createAgentSession(issueId) - Create new agent sessions on issues

LinearActivitySink:

• Wraps IIssueTrackerService to provide activity sink functionality
• Delegates to createAgentActivity() for posting activities
• Delegates to createAgentSessionOnIssue() for session creation
• Properly handles Promise-based agentSession responses

Testing

✅ 20 new unit tests covering:

• Constructor validation
• Activity posting (thought, action, response, error, elicitation)
• Session creation
• Error handling
• Multiple operations
• Workspace ID management
• Edge cases

✅ All tests passing: 410 tests in edge-worker package
✅ Build: All packages build successfully
✅ Type checking: Clean across all packages
✅ Linting: Clean (formatting auto-fixed)

Dependencies

• Builds on CYPACK-725 (GlobalSessionRegistry) from PR Create GlobalSessionRegistry for centralized session storage #766
• Base branch: cypack-724

Linear Issue

CYPACK-726: Extract IActivitySink interface and LinearActivitySink implementation