docs: brand the README hero and add community health files

[gregqualls]

Summary

Redesigns the README top section with a branded hero and adds the missing community health files. No Go code changes.

• New README hero: logo banner on pure black, subtitle, full-width demo GIF, Website / Docs / Quick start / Report a bug links, badges, and a five-capability Features list (repositories, skills, content, MCP servers, sync commands). The YAML example now matches the website homepage.
• Adds CONTRIBUTING.md (tailored to the make targets and AGENTS.md conventions), CODE_OF_CONDUCT.md (Contributor Covenant 2.1), and SECURITY.md (GitHub private reporting plus hello@getgaal.com).
• Adds a Contributing section to the README linking the three files.
• Enables blank issues and removes the Discussions contact link, since Discussions is not enabled yet, so questions can be filed as issues.

New assets: docs/assets/gaal-banner.png and docs/assets/gaal-demo.gif (optimized, 4.3 MB).

Type of Change

• Bug fix
• New feature
• Refactor (no functional change)
• Documentation
• Chore / dependency update

Related Issues

None.

Checklist

Docs and repo-meta only; no Go files changed, so the lint / build / test gates are unaffected by this PR.

• make lint passes (no Go files changed)
• make build passes on Linux and macOS (no Go files changed)
• make test-ci passes (no Go files changed)
• make coverage-ci run (not applicable, no covered code changed)
• Every new function has a slog.Debug call (not applicable, no new functions)
• Documentation updated
• No secrets or credentials are exposed

Follow-ups for maintainers

• Enable GitHub private vulnerability reporting (Settings > Security) so the SECURITY.md link works.
• Make sure hello@getgaal.com is monitored (it is the Code of Conduct and security contact).

🤖 Generated with Claude Code