chore: activate lefthook hooks and add fmt-all, mobile-fmt recipes (#751)

## Summary

- Activate lefthook pre-commit and pre-push hooks that have existed in `lefthook.yml` since the initial desktop app but were never wired up.
- Add `just hooks` recipe: sets `core.hooksPath = .hooks` and runs `lefthook install --force` to generate hook scripts.
- Wire hook installation into `scripts/dev-setup.sh` so hooks activate automatically on `just setup`.
- Enable `parallel: true` for pre-commit hooks (pre-push already had it) — all 5 format/lint checks run simultaneously.
- Add `just mobile-fmt` recipe (`dart format .`) and `just fmt-all` recipe (Rust root + Tauri Rust + Dart) as one-shot formatters.
- `.hooks/` is gitignored since lefthook generates machine-specific scripts.
- Update `AGENTS.md`: document pre-commit/pre-push hooks in Quality Gates, add `just fmt-all` and `just hooks` usage, upgrade worktree fmt gotcha from CI note to commit blocker, add `just mobile-fmt` to mobile commands; also backfill CLI-first updates (`SPROUT_AUTH_TAG`, complete exit codes, `--format compact` flag position, two new gotchas).

All hook commands delegate to `just` recipes as the single source of truth.

scripts/dev-setup.sh

@@ -210,6 +210,13 @@ else
   warn "Web directory not found at ${WEB_DIR} — skipping."
 fi
 
+# ---- Install git hooks ------------------------------------------------------
+
+log "Installing git hooks..."
+git config --local core.hooksPath .hooks
+lefthook install --force
+success "Git hooks installed"
+
 # ---- Print connection info --------------------------------------------------
 
 echo ""