Merge codex/gstack-2 into gstack2-runtime-integration

Reconcile the four integrated v2 runtime implementations (unified execution
result contract, execution profiles, capability readiness, GitHub security)
with main's browser-provider hardening.

Conflict resolutions:
- runtimeContract() generator: keep new execution-result + doctor-capability
  paragraphs, adopt main's `[matching browser flags]` fallback wording;
  regenerate the six RUNTIME.md.
- package.json: keep the strict isolated test:gstack2 runner and marked 18.0.6
  security bump; adopt main's playwright-core alias.
- bun.lock: regenerated via bun install.
- release-hardening.test.ts: adopt main's browser-provider assertions
  (resolveServerLaunchTarget, --browser managed smoke loop).

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

package.json

@@ -75,7 +75,7 @@
     "diff": "^9.0.0",
     "html-to-docx": "1.8.0",
     "marked": "^18.0.6",
-    "playwright": "^1.58.2",
+    "playwright": "npm:playwright-core@^1.58.2",
     "sharp": "^0.34.5",
     "socks": "^2.8.9",
     "xterm": "5",