feat: add environments — parametric ticket ID prefixes, env selector UI, MCP tools
- Environment model (prefix, name, description) with CRUD API
- Ticket.human_id now computed from environment prefix (e.g. PROJ-001)
- Migration 0002: environments table + backfill existing tickets to HOME env
- Router: list/create/get/patch/delete environments; list_tickets + create_ticket accept environment_id
- MCP: list_environments, create_environment tools; environment_id added to list_tickets + create_ticket
- UI: env pill selector with dropdown, new-environment modal, filter tickets by env
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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