docs: add supply-chain incident response playbook

Add a repo-level supply-chain incident response playbook for npm/GitHub Actions package-registry incidents, anchored on the May 2026 TanStack compromise and prior Shai-Hulud-style npm incidents.

- add `docs/security/supply-chain-incident-response.md` with exposure checks, immediate response steps, workflow rules, publication rules, and escalation triggers
- link the playbook from `SECURITY.md`
- reject `pull_request_target` workflows that restore or save shared dependency caches
- add a regression test for the new `pull_request_target + actions/cache` guardrail

Validation:
- node tests/ci/validate-workflow-security.test.js (12 passed, 0 failed)
- node scripts/ci/validate-workflow-security.js (validated 7 workflow files)
- npx markdownlint-cli 'SECURITY.md' 'docs/security/supply-chain-incident-response.md'
- npx markdownlint-cli '**/*.md' --ignore node_modules
- git diff --check
- node tests/run-all.js (2377 passed, 0 failed)
- GitHub CI for #1848 green across Ubuntu, Windows, and macOS

No release, tag, npm publish, plugin tag, marketplace submission, or announcement was performed.

SECURITY.md

@@ -96,5 +96,6 @@ Do not sanitize repo files in response to ephemeral reminders; they are not the
 
 - **AgentShield**: Scan your agent config for vulnerabilities — `npx ecc-agentshield scan`
 - **Security Guide**: [The Shorthand Guide to Everything Agentic Security](./the-security-guide.md)
+- **Supply-chain incident response**: [npm/GitHub Actions package-registry playbook](./docs/security/supply-chain-incident-response.md)
 - **OWASP MCP Top 10**: [owasp.org/www-project-mcp-top-10](https://owasp.org/www-project-mcp-top-10/)
 - **OWASP Agentic Applications Top 10**: [genai.owasp.org](https://genai.owasp.org/resource/owasp-top-10-for-agentic-applications-for-2026/)