Add marker-aware thread behavior

[klopez4212]

Summary

• Split from Add agent task conversation foundation #1323.
• Add marker-aware thread/timeline filtering and source-thread visibility behavior.
• Add the message row/action plumbing for continuing an agent conversation from an agent reply.
• Preserve flattened one-level replies, scroll anchoring, unread behavior, and regression tests in one reviewable layer.

Stack

1. Add agent conversation runtime foundation #1384
2. Add agent conversation domain model #1385
3. This PR
4. kennylopez-agent-tasks-conversation-screen
5. kennylopez-agent-tasks-navigation-integration

Tests

• Pre-commit hooks
• Pre-push hooks
• git diff --check 325b3fdb0d74047d6fc54bfda7c02a008af0941c..HEAD
• cd desktop && pnpm check:file-sizes && ./node_modules/.bin/tsc --noEmit