M9.T3: Error Taxonomy + Exit Codes

## Phase A3 — Error Taxonomy + Exit Codes

### Deliverables
- Structured errors: `GMIND_E_*` error codes
- Consistent exit code table in docs
- JSON mode includes `errorCode`, `hint`, `docsRef`

### Acceptance Criteria
- [ ] All failure paths produce actionable, typed errors
- [ ] CI workflows can key off error codes, not fragile strings

Part of M9 — IRONCLAD GA