refactor(gbrain): rewrite gstack-gbrain-detect bash→TS + add gbrain_local_status field

Replaces the bash detect helper with a bun shebang script sharing the
gbrain_local_status classifier from lib/gbrain-local-status.ts with the
sync orchestrator. Single source of truth for engine-status classification
between preamble-probe and orchestrator-skip paths.

Filename stays gstack-gbrain-detect (no .ts extension) so existing skill
preamble callers shell out unchanged. Shebang `#!/usr/bin/env -S bun run`
resolves bun at runtime.

Output is key/type backward-compatible with the bash version per plan
codex #5: the 9 pre-existing keys (gbrain_on_path, gbrain_version,
gbrain_config_exists, gbrain_engine, gbrain_doctor_ok, gbrain_mcp_mode,
gstack_brain_sync_mode, gstack_brain_git, gstack_artifacts_remote) stay
identical in name + type + value semantics. One new key added:
gbrain_local_status (5-state string enum).

Updates the existing schema regression at test/gstack-gbrain-detect-mcp-mode.test.ts
to include the new key. Adds test/gbrain-detect-shape.test.ts asserting
the regression contract for future changes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: garrytan