feat(worktree): add git worktree pool management for parallel agents

[whitelonng]

Summary

Adds a Git worktree pool (max 3) so multiple agents can work on the same repository on isolated branches without interfering with each other. This PR delivers the infrastructure + management UI; agent-runtime auto-integration (acquire on task start, release on task end, conflict dialog) is intentionally left for a follow-up to keep this PR reviewable.

Architecture

All git operations use the git CLI (no git2/nodegit dependency) via Kun's existing runGit() helper, which is now exported from git-service.ts.

Settings UI (settings-section-worktree.tsx)
  → window.kunGui.acquireWorktree / mergeWorktree / ...
    → ipcRenderer.invoke('worktree:xxx')
      → ipcMain.handle → worktree-service.ts
        → runGit() → git CLI

What's included

Main process (src/main/services/worktree-service.ts — new):

• acquireWorktree — create or force-reset (git reset --hard + git clean -ffd) a pool worktree. Emits a WORKTREE_HAS_CHANGES:{poolIndex}:{count} error if the worktree is dirty and force is not set — the renderer parses this to show a confirmation dialog.
• listWorktrees / removeWorktree / getWorktreeChanges / commitWorktree
• mergeWorktreeToMain — fast-forward first, then merge commit, with conflict detection (git diff --name-only --diff-filter=U)
• syncWorktreeFromMain — rebase from main with auto-stash (git stash push → git rebase → git stash pop), conflict detection
• abortMerge / continueMerge / abortRebase / cleanupWorktrees / findAvailablePoolIndex
• In-memory task ownership tracking (replaces the Rust WORKTREE_TASK_MAP)

IPC layer: 12 ipcMain.handle channels with zod-validated payloads (worktree:acquire, worktree:release, worktree:list, worktree:remove, worktree:changes, worktree:commit, worktree:merge, worktree:abort-merge, worktree:continue-merge, worktree:sync, worktree:abort-rebase, worktree:cleanup, worktree:find-available)

Preload + kun-gui-api.ts: full type-safe bridge with shared types in src/shared/worktree.ts.

Renderer:

• Zustand store (worktree-store.ts)
• Dedicated Worktrees settings section with:
  • Pool overview (main branch, in-use count, pool directory)
  • Per-pool cards showing status (in-use / idle / empty), uncommitted change count, and actions (create / release / merge / sync / remove)
  • Merge/sync result feedback with conflicted file listing
  • Cleanup-all button
• 21 i18n keys (en/zh)

Key design decisions

1. runGit exported, not duplicated — Kun's existing runGit() already handles C-locale forcing (critical for parsing git output) and timeout/buffer limits. Worktree reuses it directly.
2. reset --hard before clean -ffd — order matters: reset turns staged-new files back into untracked, then clean removes them. Reversed order leaves staged files behind.
3. WORKTREE_HAS_CHANGES as a string protocol — matches the source project's approach; the error message is parsed by the renderer to show a "N uncommitted changes, force reset?" confirmation.
4. Pool directory: ~/.kun/{project-basename}/pool-{i} by default, configurable via worktreeRoot.
5. Agent-runtime integration deferred — this PR only provides the management surface. A follow-up will hook into the agent task lifecycle to auto-acquire/release and show a conflict-resolution dialog.

Verification

• npm run typecheck passes
• npm run lint — 0 errors (9 pre-existing warnings unchanged)
• npx vitest run src/renderer/src/components — 262/262 passed

Notes

• No new npm dependencies (all git operations via CLI).
• The worktree pool is compatible with Kun's existing findNearestGitRoot (which already handles .git files for worktrees/submodules).