feat: add Botmux Desktop app shell

[anarkh]

Summary

Add the Botmux macOS Desktop app as a CLI-first desktop extension for the existing botmux runtime and dashboard.

What changed

• Add Electron desktop shell, preload bridge, renderer UI, tray/window handling, runtime controls, and log/directory helpers.
• Add CLI-first runtime ownership: the app discovers and binds the user's existing global botmux CLI/runtime instead of owning a private long-lived runtime by default.
• Add dashboard desktop compatibility probe via /__desktop/compat and desktop shell URL shaping.
• Keep Desktop installation outside the botmux CLI command surface because we do not have an Apple Developer ID / notarized distribution path yet.
• Add src/desktop/README.md and executable src/desktop/install-local.sh so users can download source, build the Desktop bundle locally, ad-hoc sign, remove quarantine, and install /Applications/Botmux.app.
• The local install script only installs the App. It does not install, upgrade, link, claim, or modify the user's global botmux CLI.
• Source install injects an effective App version into Electron packaging instead of shipping 0.0.0; git checkout uses the nearest tag, and source archives can set BOTMUX_DESKTOP_VERSION.
• Move Desktop smoke helpers from src/cli to src/desktop and keep the npm script pnpm desktop:smoke.
• Add Electron packaging config, macOS entitlements, desktop bundle scripts, and version resolution for source checkouts.
• Add focused regression tests for runtime selection, dashboard webview boundaries, IPC monitor behavior, local source install docs/script expectations, Desktop smoke, auto-start, PM2 process lookup, and packaging paths.

Scope note

This PR is intentionally separate from the stale dashboard lazy-chunk hotfix PR (#379). This PR carries the Desktop app code and its minimal CLI/dashboard integration surface. Desktop install/update is intentionally documented as source checkout + src/desktop/install-local.sh, not a botmux CLI subcommand, and the install script does not mutate the CLI installation.

Validation

Passing checks:

• CI=true pnpm install --frozen-lockfile
• pnpm exec tsc --noEmit
• pnpm exec vitest run --project unit test/desktop/*.test.ts
• pnpm build
• pnpm desktop:bundle
• bash -n src/desktop/install-local.sh
• pnpm exec vitest run --project unit test/desktop/desktop-local-install-docs.test.ts
• pnpm exec electron-builder --mac dir --config electron-builder.yml -c.extraMetadata.version=2.103.0
• bash src/desktop/install-local.sh --no-open on macOS: installed /Applications/Botmux.app, ad-hoc signed it, removed quarantine, did not open the App, and wrote Info.plist version 2.103.0.
• pnpm desktop:smoke --skip-dashboard: 6/6 checks passed after source install.

Additional full-suite check:

• pnpm exec vitest run --project unit currently fails in unrelated existing areas: test/dashboard-create-session.test.ts mock missing resolveRoleInjection, test/sandbox.test.ts opaque-dir apply assertion, and a test/whiteboard-cli.test.ts timeout in full-suite mode. Re-running the three files shows whiteboard-cli passes standalone; dashboard-create-session and sandbox remain stable unrelated failures. Desktop-focused tests above pass.