Add the CAOS desktop client

[nishu-builder]

Summary

• add a repo-scoped native Tauri desktop client over the shared CAOS conversation engine
• build and launch it with nix build .#caos-desktop, nix run .#caos-desktop, or ./result/bin/caos-desktop
• match the current durable-conversation semantics: replayed activity, interruption, interjections, forks, invitations, subagent visibility, multiplayer identity, model selection, and merge-turn publishing
• use the shared chat and workspace APIs instead of maintaining desktop-specific copies of core behavior
• provide Markdown rendering, command palette and shortcuts, plus a structured file-diff viewer

Validation

• nix build
• nix build .#caos-desktop
• npm test --prefix desktop (4 suites passed)
• cargo test --manifest-path desktop/src-tauri/Cargo.toml (4 tests passed)
• nix develop -c cargo test -p caos (69 library tests and 111 CLI/TUI tests passed)
• git diff --check

result/bin/caos-cli run-tool test reached the shared Linux builder, then stopped while compressing its dependency archive because that builder ran out of disk space (No space left on device).