fix(workspace): bound linked worktree discovery

[jonathanKingston]

Problem

Opening a persisted project inside a linked worktree can leave Copse at No folder / Loading forever when the worktree Git metadata lives on a slow or blocked external volume. Workspace activation awaits best-effort sandbox metadata discovery, and a filesystem read that never settles therefore blocks workspace:set.

Fix

• Bound automatic linked-worktree metadata discovery to two seconds.
• Abort in-flight Git metadata reads when that deadline expires.
• Keep the fallback fail-closed: the ordinary project remains allowlisted, but no linked-worktree sandbox authority is granted unless all metadata validates.
• Keep explicit worktree-manager registration strict and unbounded; only the optional open-project discovery path degrades.
• Add a regression test for a registration operation that never settles.

Validation

• pnpm test -- src/main/services/workspace.test.ts (28/28)
• pnpm run typecheck
• pnpm run build
• pnpm test:e2e --spec tests/e2e/acp-mcp-tool-name.e2e.ts (passes twice against the external-Git-dir reproduction; previously timed out at 30 seconds)
• GitHub CI check (8,544/8,544 tests)
• All eight GitHub E2E shards
• A separate local full-suite run exposed an unrelated five-second test-runner timing guard; test(exec): allow loaded runners to observe background exit #2386 isolates and fixes that flake and is independently green.

[copse-release-bot]

Reference screenshots

CI run 34299698325 published no changed screenshot candidates for 86a5a53a4e2a.
Any review PR for an older source SHA has been closed.

[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
• Light-theme syntax, fills, and selection
• 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): the external-Git-directory startup reproduction is fixed without widening sandbox authority. Timeout/abort remains limited to optional auto-discovery, incomplete discovery grants no linked-worktree metadata, and explicit worktree-manager registration stays strict. Local workspace tests passed 28/28, build passed, and the real Electron regression spec opened successfully in 8.4s. GitHub check and all eight E2E shards are now green. The unrelated local runner timing flake is isolated in #2386. No further changes needed.