fix(#270): make createWorktree() idempotent — reuse existing worktree/branch#

Problem#

createWorktree() unconditionally deleted the existing worktree directory and force-deleted the branch before recreating both. When the branch was already checked out in another worktree, git branch -D failed silently and left the task stuck in a permanent retry loop.

Task / Link#

GitHub Issue #270 — AgentSDE/agent-core

Changes#

• Added isValidWorktree(path, repoPath) private helper — checks git worktree list --porcelain to detect registered worktrees
• Replaced destructive cleanup block with three-path idempotent logic:
  • Valid worktree exists → reuse it (git fetch origin + git reset --hard origin/master)
  • Directory exists but invalid → remove directory, fall through to branch check
  • Branch exists locally, no worktree → git worktree add <path> <branch> (no -b)
  • Neither exists → fresh git worktree add -b <branch> <path> origin/master
• Removed git branch -D block entirely (root cause of stuck loop)
• Added 4 unit tests covering all idempotent paths

Notes#

git reset --hard origin/master in the reuse path discards any uncommitted changes in the worktree — acceptable since agent worktrees are always ephemeral and start from origin/master.

Testing#

• 15 unit tests pass (npm run test)
• npm run lint passes with zero errors in changed files