refactor(framework): thin-core prompt diet — cut injected contract ~53%

Thin defaults/AGENTS.md + defaults/TOOLS.md + runtime/claude/RUNTIME.md to a core
that references guides on demand; preserve the full tool catalog in new
guides/TOOLS-REFERENCE.md. All 12 hard gates verbatim; validated via a deterministic
gate-checklist + a 9-probe headless A/B (thin 7/9 vs monolith 5/9). Composed
AGENTS+TOOLS+RUNTIME 8,827->4,122 tok. Diet-only against repo content; no behavioral
change, nothing imported from drifted deployments.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/TASKS.md

@@ -39,3 +39,8 @@ Active workstream is **W1 — Federation v1**. Workers should:
 1. Read [docs/federation/MISSION-MANIFEST.md](./federation/MISSION-MANIFEST.md) for workstream scope
 2. Read [docs/federation/TASKS.md](./federation/TASKS.md) for the next pending task
 3. Follow per-task agent + tier guidance from the workstream manifest
+
+## Thin-core prompt diet (#528) — feat/contract-thin-core
+- Status: PR open, awaiting maintainer merge ratification (fleet-governing change).
+- Cut always-injected contract AGENTS+TOOLS+RUNTIME 8,827→4,122 tok (−53%); all 12 hard gates intact.
+- Validation: deterministic gate-checklist PASS; headless A/B thin 7/9 vs monolith 5/9. Detail: scratchpads/contract-thin-core.md.