chore: share agent config across tools

[fullstackjam]

What does this PR do?

Makes AGENTS.md the canonical agent guide, shares repo skills through
.agents/skills, and exposes the same skills to Claude Code with relative
symlinks under .claude/skills.

Why?

Codex and Claude Code previously used separate instruction and skill paths,
which made the repository vulnerable to duplicated guidance and configuration
drift. This gives both tools one maintained source of truth and adds an
architecture test that keeps the adapters valid.

Testing

• go vet ./... passes
• Relevant tests added or updated
• Tested locally (go test ./internal/...)

Cross-repo checklist

• No docs/content update is needed in openboot.dev
• No CLI ↔ server API contract change

Notes for reviewer

• Adds the repo-scoped ship-pr skill from fullstackjam/skills.
• .agents/skills is canonical; .claude/skills/*/SKILL.md entries are
  portable relative symlinks.