feat: add workspace init and migration flow

[ThomsenDrake]

Stack position

This is PR 2/5 in the upstream contribution stack.

Depends on:

• Upstream PR #28

GitHub only allows stacked base branches inside the same repository, so this review slice targets the previous branch on the fork. After PR1 lands, this can be retargeted upstream.

Summary

• harden workspace resolution and startup guardrails
• add desktop init and migration workflows plus Tauri command support
• add frontend /init flow and WorkspaceInitGate

Out of scope

• runtime/state foundation from PR1
• audio transcription
• Chrome DevTools MCP
• provider, auth, or config changes

Notes

• I have not rerun the full PR-specific test suite as part of PR creation