feat: createWorktreeForJob helper — isolate agent per job in git worktree

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/git/worktree.ts

@@ -0,0 +1,57 @@
+import { execFile } from 'node:child_process'
+import { promisify } from 'node:util'
+import * as path from 'node:path'
+import * as os from 'node:os'
+import * as fs from 'node:fs/promises'
+
+const exec = promisify(execFile)
+
+async function branchExists(repoRoot: string, name: string): Promise<boolean> {
+  try {
+    await exec('git', ['show-ref', '--verify', '--quiet', `refs/heads/${name}`], { cwd: repoRoot })
+    return true
+  } catch {
+    return false
+  }
+}
+
+export async function createWorktreeForJob(opts: {
+  repoRoot: string
+  jobId: string
+  branchName: string
+  baseRef?: string
+}): Promise<{ worktreePath: string; branchName: string }> {
+  const { repoRoot, jobId, baseRef = 'origin/main' } = opts
+  let { branchName } = opts
+
+  const parent =
+    process.env.SCRUM4ME_AGENT_WORKTREE_DIR ??
+    path.join(os.homedir(), '.scrum4me-agent-worktrees')
+
+  await fs.mkdir(parent, { recursive: true })
+
+  const worktreePath = path.join(parent, jobId)
+
+  // Reject if worktree path already exists — caller must remove it first
+  try {
+    await fs.access(worktreePath)
+    throw new Error(
+      `Worktree path already exists: ${worktreePath}. Call removeWorktreeForJob first.`,
+    )
+  } catch (err: unknown) {
+    if ((err as NodeJS.ErrnoException).code !== 'ENOENT') throw err
+  }
+
+  await exec('git', ['fetch', 'origin', '--prune'], { cwd: repoRoot })
+
+  // Suffix with timestamp when branch already exists
+  if (await branchExists(repoRoot, branchName)) {
+    branchName = `${branchName}-${Date.now()}`
+  }
+
+  await exec('git', ['worktree', 'add', '-b', branchName, worktreePath, baseRef], {
+    cwd: repoRoot,
+  })
+
+  return { worktreePath, branchName }
+}