fix(desktop): stop looping macOS TCC permission prompts#2745

Summary

Fixes pingdotgg/t3code#2254 Fixes pingdotgg/t3code#2737 Fixes pingdotgg/t3code#2088

Three issues with the same shape: macOS TCC permission prompts ("Other apps", "Music", "Documents", "Downloads", etc.) re-fire repeatedly because three different code paths trigger fresh file access on every render or panel mount, with no caching or enable-gate.

Repro / root causes:

• Tailscale, "Other apps" category (issues 2254, 2737). DesktopServerExposure.getAdvertisedEndpoints unconditionally spawned tailscale status whenever the Connections settings panel mounted, even when the user had not opted into any network exposure. On Mac App Store Tailscale the CLI lives inside Tailscale's sandbox container, so each spawn re-fires the "T3 Code would like to access data from other apps" TCC prompt. Issue 2737's reproduction (enable Network Access → prompt loops every few seconds) maps directly to the Settings panel re-firing this effect.
• Path-autocomplete, Music / Documents / Downloads / Desktop categories (issues 2254, 2088). CommandPalette prefetched the highlighted browse child on every render (useEffect keyed on highlightedBrowseEntry). Each prefetch sent partialPath: "~/Music/" (with trailing separator from appendBrowsePathSegment) to WorkspaceEntries.browse, which readdir'd the gated dir server-side. Arrow-keying past Music, Documents, Downloads, Desktop, Movies, Pictures in the home folder fired one TCC prompt per highlight change.
• Denied TCC prompt re-fires. WorkspaceEntries.browse surfaced EACCES/EPERM from readdir as a WorkspaceEntriesBrowseError, which the renderer's React Query then re-attempted on retry — re-firing the prompt.

Changes

• apps/desktop/src/backend/DesktopServerExposure.ts — early-return from getAdvertisedEndpoints with only the core endpoints when mode !== "network-accessible" and tailscaleServeEnabled === false. Wrap readTailscaleStatus in Effect.cachedWithTTL (60s) so even when the gate passes, panel remounts don't re-spawn the CLI.
• apps/desktop/src/backend/tailscaleEndpointProvider.ts — accept an optional readMagicDnsName Effect so the caller can inject the cached reader. Default path is unchanged.
• apps/web/src/components/CommandPalette.tsx — drop the highlighted-child prefetch; keep the parent prefetch for back-navigation. The user pays one readdir when they actually navigate into a directory instead of one per arrow-key press.
• apps/server/src/workspace/Layers/WorkspaceEntries.ts — catch EACCES/EPERM from readdir in browse and return an empty listing so a denied prompt stops re-firing on retry.

Tests

• DesktopServerExposure.test.ts: new test using a "die on spawn" ChildProcessSpawner layer asserts that getAdvertisedEndpoints does not invoke the spawner while the server is in local-only mode with Tailscale Serve off.
• tailscaleEndpointProvider.test.ts: new test verifies the readMagicDnsName injection bypasses readTailscaleStatus and produces the magic-DNS endpoint as expected.
• WorkspaceEntries.test.ts: new test mocks fsPromises.readdir to reject with EACCES and asserts browse returns { parentPath, entries: [] } instead of erroring.

Test plan

• bun typecheck clean
• bun lint clean (no new warnings)
• bun fmt clean
• bun run test — 1023 passed, 4 skipped (matches main), including the three new tests
• On macOS with Mac App Store Tailscale installed: open Settings → Connections without enabling Network Access, confirm no "Other apps" prompt appears
• With Network Access enabled, confirm the prompt fires at most once per 60s instead of on every panel mount
• In the command palette browse mode, arrow-key past Music / Documents / Downloads and confirm no TCC prompts fire until you press / to navigate into one

Note: I do not have a Mac App Store Tailscale install to verify the macOS prompt behavior end-to-end; the fix targets the code paths identified in the issues and is covered by unit tests for the gating, caching, and EACCES handling.