fix(dev): support cross-device Electron cache setup

[jonathanKingston]

Summary

• fall back to a staged recursive copy when promoting Electron's extracted dist returns EXDEV
• keep the destination atomic by copying beside the shared cache and renaming only after completion
• preserve the source and clean incomplete staging directories when copying fails

Validation

• pnpm test -- move-directory dev-bundle-path agent-service roadmap-issue-import provider-selection
• pnpm run check (8,547 tests passed)

[github-actions]

🖥️ PR preview

• Marketing site
• Browser demo
  Interactive, with a mocked backend — jump to a scenario:
• Builds a cupcake site
• Agent-proposed edits open the Changes panel
• Markdown list indentation
• Responsive composer footer
• Subagent display visual reference
• Subagent stays ungrouped beside reading tools
• Semantic search subagent markdown
• Light-theme approval with a bright accent
• Grouped outside-sandbox command approval
• Remote desktop discovered-port list with one selected
• Settings scroll + sticky footer geometry
• Per-model reasoning / temperature / top-p
• Thread switch hydration notice
• Thread switch hydration failure
• Chat layout styling
  _{Static demo of the renderer against a mock API — the browser pane, terminal, and all main-process IPC are stubbed, so it never proves main-process behaviour. Rebuilt whenever a push changes the demo bundle; these links go live once the queued deploy lands, shortly after this comment. Removed when the PR closes.}

[jonathanKingston]

Validation follow-up (2026-09-05): re-reviewed this diff against the current origin/main. The direct rename remains the fast path; only EXDEV falls back to destination-side staging, the staging directory is promoted atomically, the source is removed only after promotion, and failed copies retain the source while cleaning staging. The PR is mergeable and every required CI check is green. No further changes needed.