Add project filtering support for milestone queries

[AdiKsOnDev]

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:

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)