Add ensureSession to recreate missing worktrees

src/commands/helpers.ts

@@ -1,5 +1,6 @@
-import { join } from "path"
-import { unlink } from "fs/promises"
+import { basename, join } from "path"
+import { homedir } from "os"
+import { mkdir, symlink, unlink } from "fs/promises"
 import { $ } from "bun"
 import * as git from "../git.ts"
 import * as vm from "../vm.ts"
@@ -18,6 +19,40 @@ export async function requireSession(branch: string): Promise<{ root: string; se
   return { root, session }
 }
 
+/** Look up a session by branch, recreating the worktree/session if the branch exists but the session doesn't. */
+export async function ensureSession(branch: string): Promise<{ root: string; session: Session }> {
+  const root = await git.repoRoot()
+  const existing = await state.getSession(root, branch)
+  if (existing) return { root, session: existing }
+
+  // No session — check if the branch exists
+  const exists = await git.branchExists(branch, root)
+  if (!exists) {
+    die(`No session or branch found for "${branch}".`)
+  }
+
+  // Recreate worktree and session
+  const worktreeAbs = join(homedir(), '.sandlot', basename(root), branch)
+  try {
+    await git.createWorktree(branch, worktreeAbs, root)
+    await mkdir(join(root, '.sandlot'), { recursive: true })
+    await symlink(worktreeAbs, join(root, '.sandlot', branch))
+  } catch (err) {
+    // Clean up on failure — but do NOT delete the branch (it already existed)
+    await git.removeWorktree(worktreeAbs, root).catch(() => {})
+    await unlink(join(root, '.sandlot', branch)).catch(() => {})
+    die(`Failed to recreate session: ${(err as Error).message ?? err}`)
+  }
+
+  const session: Session = {
+    branch,
+    worktree: worktreeAbs,
+    created_at: new Date().toISOString(),
+  }
+  await state.setSession(root, session)
+  return { root, session }
+}
+
 /** Tear down a session: clear activity, remove worktree, unlink symlink, remove state. */
 export async function teardownSession(root: string, branch: string, worktree: string): Promise<void> {
   await vm.clearActivity(worktree, branch)

src/commands/open.ts

@@ -2,14 +2,14 @@ import * as vm from "../vm.ts"
 import * as state from "../state.ts"
 import { spinner } from "../spinner.ts"
 import { renderMarkdown } from "../markdown.ts"
-import { requireSession, saveChanges } from "./helpers.ts"
+import { ensureSession, saveChanges } from "./helpers.ts"
 
 export async function action(
   branch: string,
   prompt: string | undefined,
   opts: { print?: string; save?: boolean },
 ) {
-  const { root, session } = await requireSession(branch)
+  const { root, session } = await ensureSession(branch)
 
   const effectivePrompt = opts.print || prompt
   if (effectivePrompt) {