[feature] Panel per-cell project picker + pnpm format infra

MegaTask root-subtasks can fan out across cells (be+fe, fe+uxui). Since a
RoboCo project is per-cell (ProjectTable.assigned_cell), a monorepo is N
per-cell projects sharing one git_url — so multi-cell IS multi-project. The
batch-review card now shows one project Select per the_work entry, scoped to
that cell's repos, instead of one Select bound to a single top-level
project_id. confirmBatch validates each cell's project is in scope and the
batch still spans >=2 distinct projects.

- prompter.ts: CellWork gains optional project_id (the per-cell picker seam).
- batch-review-card.tsx: per-cell Selects (one per the_work entry), scoped to
  the cell's projects; legacy single-cell drafts keep the one-Select path.
- use-prompter.ts: updateBatchDraftProject edits per-cell (entryIndex);  confirmBatch validates every cell; batchFromEvent parses per-cell map.

Also adds the missing pnpm format infrastructure (the panel had no formatter
at all): prettier devDep + .prettierrc.json (default-style config: 80-col,
double-quote, semi, trailing-comma-all) + .prettierignore, plus format /
format:check scripts. Only the 3 changed files above were reformatted; the
~222 pre-existing non-compliant files are left untouched (a wholesale reformat
is a separate explicit decision, not bundled into this feature).

panel/package.json

@@ -10,7 +10,9 @@
     "lint": "eslint",
     "typecheck": "tsc --noEmit",
     "test": "vitest run --coverage",
-    "test:watch": "vitest"
+    "test:watch": "vitest",
+    "format": "prettier --write .",
+    "format:check": "prettier --check ."
   },
   "dependencies": {
     "@dnd-kit/core": "^6.3.1",
@@ -66,6 +68,7 @@
     "eslint": "^9",
     "eslint-config-next": "16.1.1",
     "jsdom": "^29.1.1",
+    "prettier": "^3.8.5",
     "tailwindcss": "^4",
     "tw-animate-css": "^1.4.0",
     "typescript": "^5",