feat(framework): P3 — extract Constitution (L0) + gut AGENTS dispatcher

Splits the 155-line thin-core AGENTS.md into:
- defaults/CONSTITUTION.md (L0): gates + integrity + escalation + block-vs-done
  + mode + two-axis precedence + hooks-are-the-gate + framework-PR firewall +
  structured-reasoning capability + tier-aware self-load. Capability-verb authored.
- defaults/AGENTS.md gutted to an ~80-line load-order dispatcher + guide table
  (kills the false "already in context, do not re-read" line).
- constitution/LAYER-MODEL.md: source-only governance spec (layers + precedence).

Non-regression wiring (fresh-install functional; upgrade-safety is P4):
- launch.ts injects CONSTITUTION.md before AGENTS.md (tolerant of un-reseeded installs)
- install.sh + file-adapter.ts seed CONSTITUTION.md (+ test fixture updated)

Runtime adapters: capability-verb the sequential-thinking binding; claude/codex/
opencode restate the REQUIRED hard-stop, pi binds to native thinking (gate=false)
— restores the force the adversarial review flagged as weakened.

Gate hardening (dual-engine review): identity denylist now covers examples/
(closes the Codex open-source gap), self-test-first, *.json in scope, ci.yml
typecheck depends on sanitization (fail-fast), L0 line-count ceiling (<=120).

Adversarial gate-preservation review: every original rule traced to L0, the
dispatcher, or a routed guide — nothing lost.

Refs #542, closes #574

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

packages/mosaic/framework/runtime/claude/RUNTIME.md

@@ -7,7 +7,7 @@ Claude-runtime behavior only. Global rules win if anything here conflicts.
 1. Follow the Session Start load order in `~/.config/mosaic/AGENTS.md`.
 2. Runtime config lives in `~/.claude/settings.json` (hooks, model, plugins, permissions) and
    `~/.claude/hooks-config.json`.
-3. sequential-thinking MCP is required.
+3. Structured reasoning (Constitution) binds to the sequential-thinking MCP on this harness; it is REQUIRED — if unavailable, report the failure and stop planning-intensive execution.
 4. First response MUST declare mode per the global contract.
 5. Git wrappers first for issue/PR/milestone ops; runtime-default confirmation prompts do NOT
    override Mosaic hard gates (push/merge/issue-close without routine confirmation).