Add project filtering support for milestone queries

Enable filtering milestones by project ID in GraphQL queries.

**Current behavior:**
Project filtering is disabled in `build_milestone_filter()` due to GraphQL schema issues. Client-side filtering is used as a workaround in `resolve_milestone_id()`.

**Example usage:**
```bash
linear milestone list --project "Backend Improvements"
```

**Implementation:**
- Research correct GraphQL filter syntax for milestone project filtering
- Update `build_milestone_filter()` to support project_id parameter
- Remove TODO comment in queries.py
- Test with Linear API to ensure schema compatibility

**Files to modify:**
- `src/linear_cli/api/queries.py` (build_milestone_filter function)