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/settings.json

@@ -28,7 +28,6 @@
       "Bash(git rev-parse --show-toplevel)",
 
       "Bash(tooling/db/ticket *)",
-      "Bash(tooling/db/sprint *)",
       "Bash(tooling/db/sqlite-query *)",
       "Bash(tooling/db/sqlite-exec *)",
       "Bash(tooling/db/sqlite-init)",
@@ -58,22 +57,15 @@
       "Bash(ruff check)",
       "Bash(tests/run-*)",
 
-      "Bash(mkdir -p docs/sprints/*)",
-      "Write(docs/sprints/*)",
-
       "Bash(chmod *)",
       "Bash(ls *)",
       "Bash(find *)",
       "Bash(list *)",
       "Bash(tree *)",
       "Bash(sed -n *)",
-      "Bash(.claude/skills/sprint-start/scripts/start-sprint.sh *)",
-      "Bash(.claude/skills/sprint-start/scripts/sprint-teardown.sh *)",
-
       "Skill(git-commit)",
-      "Skill(sprint-start)",
-      "Skill(sprint-plan)",
-      "Skill(pr-push)",
+      "Skill(whats-next)",
+      "Skill(pr-process)",
       "Skill(pr-review)",
       "Skill(ticket)",
       "Skill(docs-search)",