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/stig.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).
 
 ## Status
 **ACTIVE.** v0.2 brings a character customisation UI requirement and removes the dual-character variant for insert/HUD.
@@ -49,7 +49,7 @@ See `docs/DEVOPS.md` for full procedures. Your key targets:
 - `make setup` — verify Godot is installed
 - `make client` — launch Godot with `client/` project
 - `make test-client` — gdUnit4 tests (headless runner pending)
-- `make lint-client` — gdlint/gdformat (pending setup)
+- `make lint-client` — gdlint/gdformat
 - `make ci-client` — full client CI pipeline (lint -> build -> test)
 - Client code lives in `client/` — scenes, GDScript, assets
 - UI tests use gdUnit4 (D-030)