Add agent conversation runtime foundation

[klopez4212]

Summary

• Split from Add agent task conversation foundation #1323.
• Add the runtime/protocol foundation for continued agent conversations.
• Add the new conversation event kind sync and desktop/mobile model constants.
• Thread auth-tag/profile reconcile plumbing through ACP and Tauri runtime paths.

Stack

1. This PR
2. kennylopez-agent-tasks-domain-model
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