docs(briefings): extract shared PROJECT_STATE.md, purge stale boilerplate

Create PROJECT_STATE.md at repo root with cascade phase table,
decision count, and current sprint. All 18 briefings now reference
this file instead of duplicating the project state paragraph.

Also removes stale content from briefings:
- "v0.2 direction: generator-first" (contradicts D-166 cascade)
- "Sprint 25 generator spike confirmed as critical path" (Phase 5)
- Agent-specific generator prerequisite claims
- Gestalt "v0.2 Pivot" section reframed as "Key Design Pivots"

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

docs/briefings/si.md

@@ -11,7 +11,7 @@ updated: 2026-03-13
 Last updated: 2026-03-13
 
 ## Current Project State
-The Settled Reach: top-down immersive sim, single-character perspective, asymmetric information core mechanic, Rimworld-style storyteller. 137+ confirmed decisions. Where's the Fun? Workshop complete (2026-03-05) — 24 decisions locked. v0.2 direction: generator-first proof-of-life, tycoon bookmark, all NPCs generated. Sprint 25 generator spike confirmed as critical path.
+See [PROJECT_STATE.md](../../PROJECT_STATE.md) for shared project state (cascade phase, sprint, decision count).
 
 ## Decisions Relevant to Your Role
 All decisions across all domains. Read `decisions/README.md` for the index. Query decisions DB for sprint planning:
@@ -24,8 +24,8 @@ make decisions-active     # List all active decisions
 ## Open Questions Assigned to You
 All open questions - tracks as tickets for assignment and progress monitoring.
 
-## v0.2 Critical Path (from Where's the Fun? Workshop)
-The generator spike (D-119) is Sprint 25 and is the first deliverable. Everything gates on it.
+## Development Cascade (D-166)
+Development follows a strict 6-phase cascade. Currently in Phase 1 (wiki content). Generator spike (D-119) is Phase 5.
 
 **Prerequisites that must exist before/during the generator spike:**
 1. Miri: Zone identity spec + Van Maanen's Star culture profile — blocks voice, NPC gen, AI pipeline