[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/.prettierrc.json

@@ -0,0 +1,10 @@
+{
+  "semi": true,
+  "singleQuote": false,
+  "trailingComma": "all",
+  "printWidth": 80,
+  "tabWidth": 2,
+  "arrowParens": "always",
+  "bracketSpacing": true,
+  "endOfLine": "lf"
+}