Add agent conversation domain model

[klopez4212]

Summary

• Split from Add agent task conversation foundation #1323.
• Add desktop parsing/building/storage helpers for focused agent conversations.
• Add recap/title derivation helpers and model-level regression coverage.
• Add relay filter support for fetching task marker state.

Stack

1. Add agent conversation runtime foundation #1384
2. This PR
3. kennylopez-agent-tasks-thread-behavior
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