feat: add customer management commands

[eslerm]

Add customer/CRM management: customers, needs, tiers, and statuses.

Scope

• 8 queries across 4 sub-entities (customer, need, status, tier)
• 18 mutations including merge, upsert, and attachment-based need creation
• New command group with ~16 subcommands
• Full filter support for customer list
• Estimated ~2,500 LOC

Open Questions

• What Linear plan level is required for customer features?
• Should customer merge require --confirm?
• Implement customerUpsert or just create/update?
• Split into multiple PRs? (Phase 1: CRUD, Phase 2: needs, Phase 3: config)

See planning/05-customers.md