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/team-patterns.md

@@ -3,7 +3,7 @@
 ## Model selection
 
 Default model is Opus 4.6 (200K context). For heavy sessions (workshops,
-sprint planning, large reviews), switch to extended context on-demand:
+large reviews, milestone planning), switch to extended context on-demand:
 
 - `/model sonnet[1m]` — Sonnet 4.6 with 1M context window
 - `/model opus[1m]` — Opus 4.6 with 1M context window
@@ -20,7 +20,7 @@ Key: writers use Write tool directly (no transcription bottleneck), librarian ca
 
 ## Team monitoring (stuck agent detection)
 
-When leading a team (sprint, workshop, or any multi-agent session):
+When leading a team (workshop, batch, or any multi-agent session):
 
 **Agent heartbeat rule** — include in every agent spawn prompt:
 > If you have been working on a single task for more than 15 minutes