feat(desktop): add per-agent working directories

[iminierai-aig]

Summary

Adds an optional machine-local working directory to each Desktop-managed agent so its buzz-acp process can start in an explicitly selected workspace instead of always using the shared Buzz nest.

• validates and canonicalizes configured directories through one backend path
• fails closed for missing, inaccessible, relative, root, or non-directory paths
• applies the effective directory to the managed buzz-acp parent process
• includes the effective directory in spawn configuration hashing so changes trigger existing restart-drift behavior
• preserves the current Buzz nest default for records without the field
• excludes machine-local paths from Persona/Team snapshots and managed-agent relay events
• adds an Advanced editor field with explicit workspace-guardrail language

This is working-directory enforcement only. It is not a sandbox, credential boundary, or filesystem isolation mechanism.

Related issue

Addresses the optional per-agent working-directory portion of #3364. #3822 describes broader channel-scoped/runtime-portable workspace routing, which remains out of scope.

No duplicate implementation PR was found.

Testing

After rebasing onto main at 06b60e6:

• 999 managed-agent Tauri tests passed
• full Tauri run: 2,236 passed and 13 ignored; one unrelated timing-sensitive Codex version-probe test failed once and passed on immediate isolated rerun
• Tauri Clippy passed for all targets/features with warnings denied
• 4,372 frontend tests passed
• all 8 edit-agent.spec.ts Playwright tests passed, including working-directory save/reopen persistence
• TypeScript typecheck, Biome, Rust formatting, file-size/text/pubkey guards, E2E build, and git diff --check passed

Before the rebase, an isolated x86_64 macOS app bundle also built with a development-only identifier and completed a 12-second startup smoke test without fatal errors. The relevant checks were run separately rather than through one aggregate just ci invocation.

UI

Advanced agent settings expose the optional machine-local path and state the security boundary explicitly.