Add SessionStart hook to auto-install deps

[leap21ai]

What

Two things that make the repo turnkey to run on your own machine (laptop / Mac mini) instead of a cloud sandbox:

1. A SessionStart hook that auto-installs the pnpm workspace before the agent starts (web and claude remote-control sessions).
2. A docs/local-setup.md capturing the local dev workflow + Remote Control steps.

Changes

• .claude/hooks/session-start.sh — enables corepack, pins pnpm@8.15.9 (matching packageManager), runs pnpm install. Idempotent, non-interactive.
• .claude/settings.json — registers the hook on SessionStart.
• .gitignore — narrows the .claude rule so the shared hook + settings.json are tracked while settings.local.json stays ignored.
• docs/local-setup.md — prerequisites (Node 22+, pnpm via corepack), install/verify, AI key, and Remote Control (steer a local session from the Claude app, no GitHub needed). Linked from the README docs list.

Validation

Ran in this environment:

• ✅ Hook: pnpm install completed (234 packages). Also confirmed live — the hook ran cleanly on a session resume.
• ✅ Lint: biome check on a core source file — clean.
• ✅ Test: vitest run on packages/core/src/__tests__/paths.test.ts — 11/11 passed.

Notes

• Hook runs synchronously — guarantees deps are ready before the session starts (no race), at the cost of slightly slower startup. Can switch to async if preferred.
• Once merged to the default branch, all future sessions pick it up.
• The "Choose environment" picker in the app is cloud-only and always uses GitHub; Remote Control (claude remote-control) is the path that runs on your own hardware — documented in docs/local-setup.md.