chore: finish the Guard-Core org migration (docs, metadata, links, demo container)

[rennf93]

Summary

Finishes the Guard-Core org migration for this repo, plus the two guard-agent extras: the packaged SKILL.md link fix and a new container release workflow.

Categories of changes

• README.md + docs/index.md: hero link and legend img, five workflow badge href+img pairs, shields last-commit, Docs links, cross-doc links moved to guard-core.github.io (path suffixes preserved); new Docs badge pointing at the new Pages URL in the same shields style
• mkdocs.yml: site_url, repo_name, repo_url, social link, announcement issues link, og:image (personal author profile URL left untouched)
• pyproject.toml: all four project.urls entries moved to the org and the new Pages host; author email renamed to rennf93@guard-core.com
• shim/pyproject.toml + shim/README.md: Repository, Documentation and CHANGELOG links; author email
• guard_agent/.agents/skills/guard-agent/SKILL.md: all seven ecosystem links (this is the copy that ships inside the wheel under guard_agent/, confirmed present in a fresh wheel build)
• docs/adapters/: fastapi.md, flask.md, tornado.md, django.md sibling repo and cross-doc links
• docs/installation.md git clone URL, docs/robots.txt sitemap
• .github/ISSUE_TEMPLATE/config.yml, SECURITY.md, AGENTS.md, CLAUDE.md, CODE_OF_CONDUCT.md (enforcement contact -> contact@guard-core.com), FUNDING.yml (github: Guard-Core, paypal line and everything else kept), context7.json (now https://context7.com/Guard-Core/guard-agent)
• CHANGELOG.md + docs/release-notes.md history entries left untouched

Container workflow (new)

• examples/demo_app/: minimal FastAPI demo that initializes GuardAgentHandler from env vars (GUARD_AGENT_ENDPOINT, GUARD_AGENT_API_KEY, GUARD_AGENT_PROJECT_ID), emits a test SecurityEvent at startup, and serves /, /health and POST /events. Starts and serves with no external services required (telemetry buffers and degrades gracefully when the ingestion endpoint is unreachable)
• examples/demo_app/Dockerfile: two-stage uv build mirroring the family Dockerfile shape, non-root user, OCI labels including the package description
• .github/workflows/container-release.yml: follows the Guard-Core/flaskapi-guard family pattern; triggers on push to master, workflow_dispatch and release; builds and pushes ghcr.io/guard-core/guard-agent:latest always and :version on release; logs in with GITHUB_TOKEN

Verification

• uv run ruff format --check + ruff check on touched files: pass
• uv run mypy . (repo standard, whole repo): Success, no issues in 56 source files
• uv run mkdocs build --strict: pass, no new errors
• docker build of examples/demo_app/Dockerfile: pass with zero warnings; container run verified /, /health (healthy) and POST /events with no external services, and env var overrides reflected at runtime
• tomllib parse of pyproject.toml + shim/pyproject.toml: pass
• git grep -n rennf93: 113 before -> 9 after. The remaining 9 are: 4 CHANGELOG/release-notes history lines, 1 mkdocs personal author profile, 2 rennf93/github-actions-secrets-mgmt@v1.5 workflow action refs (the shared reusable action still lives under the personal namespace and the migrated siblings reference it there), and 2 mandated author emails (rennf93@guard-core.com)