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/nigel.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).
 
 ## v0.2 Pivot — Key Changes for Your Role
 The replayability frame shifts from "same conspiracy, different character lens" to "same generator, different seeded world." The anti-metagaming goal survives; the mechanism changes.
@@ -38,7 +38,7 @@ The replayability frame shifts from "same conspiracy, different character lens"
 - D-029: Variable entanglement rate per seed — economic/social triangles in v0.2
 - D-114: v0.2 proof-of-life = generator + graphics
 - D-117: Tycoon bookmark, zero investigation content
-- D-119: Generator spike Sprint 25 — critical path
+- D-119: Generator spike — Phase 5 per D-166 cascade (not imminent)
 - D-122: All NPCs generated
 - D-127: Player choices are the content — Rimworld model
 - D-136: Both structural and cosmetic generator variety at different scales