Wire agent conversations into navigation

[klopez4212]

Summary

• Split from Add agent task conversation foundation #1323.
• Wire focused agent conversations into AppShell, channel unread routing, and sidebar navigation.
• Add the sidebar task children UI and final integration polish.
• This branch completes the split stack and is byte-equivalent to the original Add agent task conversation foundation #1323 head when stacked on the previous PRs.

Stack

1. Add agent conversation runtime foundation #1384
2. Add agent conversation domain model #1385
3. Add marker-aware thread behavior #1386
4. Add agent conversation screen #1387
5. This PR

Tests

• Pre-commit hooks
• Pre-push hooks
• git diff --check 325b3fdb0d74047d6fc54bfda7c02a008af0941c..HEAD
• cd desktop && pnpm check:file-sizes && ./node_modules/.bin/tsc --noEmit
• cd desktop && pnpm test
• cargo test --manifest-path desktop/src-tauri/Cargo.toml deep_link --lib