feat: add project label, relation, and status management

[eslerm]

Extend project commands with label CRUD, inter-project relations, and project status management.

Scope

• 4 new queries: projectLabel, projectLabels, plus status/relation queries
• 12 new mutations: label CRUD + linking, relation CRUD, status CRUD
• Extends existing project command group
• Estimated ~1,600 LOC

Subcommands

• project label-list, label-create, label-update, label-delete, add-label, remove-label
• project relation-create, relation-delete
• project status-create, status-update, status-archive

Open Questions

• What relation types are available? (blocks, depends-on, related-to?)
• Should projectLabelRetire be separate from label-delete?
• Split into multiple PRs? (labels, relations, statuses)

See planning/08-project-labels-relations.md