docs: Update internal changelog for IActivitySink extraction (CYPACK-726)

Add changelog entry documenting the extraction of IActivitySink interface
and LinearActivitySink implementation.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: cyrusagent

CHANGELOG.internal.md

@@ -6,6 +6,7 @@ This changelog documents internal development changes, refactors, tooling update
 
 ### Added
 - Created `GlobalSessionRegistry` class for centralized session storage across all repositories, enabling cross-repository session lookups in orchestrator workflows ([CYPACK-725](https://linear.app/ceedar/issue/CYPACK-725), [#766](https://github.com/ceedaragents/cyrus/pull/766))
+- Extracted `IActivitySink` interface and `LinearActivitySink` implementation to decouple activity posting from `IIssueTrackerService`, enabling multiple activity sinks to receive session activities ([CYPACK-726](https://linear.app/ceedar/issue/CYPACK-726), [#767](https://github.com/ceedaragents/cyrus/pull/767))
 
 ## [0.2.13] - 2026-01-15