feat(meta): replace sprint workflow with kanban + milestones (D-221)

Sprint-based workflow (38 sprints) replaced by kanban + milestones.
Milestones are many-to-many with tickets and can block each other.

New: /whats-next skill (dependency-driven batch selection with Si
refinement review), /pr-process skill (renamed from pr-push, adds
review comment pickup), clerk agent + pre-push hook for D-record
consistency checks.

Deleted: sprint CLI, sprint-start/sprint-plan/sprint-status skills,
team-scoped file restrictions. Si rewritten as refinement manager.
All 19 agent briefings updated from stale PROJECT_STATE.md reference
to live ticket milestone queries.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.claude/rules/project-structure.md

@@ -21,15 +21,14 @@ docs/
   architecture/       # Technical architecture documents
   design/             # Game design documents
   diagrams/           # d2 source + PNG renders
-  sprints/            # Sprint briefings per team
+  sprints/            # Historical archive (Sprint 1–38) — no new sprint directories
   workshops/          # Workshop briefs and outputs
 db/
   schema.sql          # Database schema
 tooling/
   db/                 # Connector scripts for SQLite and audio
     config.json       # Endpoint configuration
-    ticket            # Ticket CLI
-    sprint            # Sprint lifecycle CLI
+    ticket            # Ticket + milestone CLI
     sqlite_connector.py   # SQLite mini MCP
     audio_connector.py    # Stable Audio Open connector
 .claude/