Port all referenced skills, add deslop, enforce reference checks in CI

[Cloeille]

Follow-up to #1 / #2. PR #2 was merged after its first commit only; the three commits below were pushed afterwards and never reached main.

What this adds

• Ports every skill, playbook and asset the guide/README referenced but that was missing (48 skills now, incl. the 20 principle-* skills, why, tdd, teach, recall, reflect, create/maintain-verification-skill, no-comments, show-me-your-work, benchmark-checklist, and 10 playbooks), adapted from Cursor to Hermes (delegate_task, clarify, session_search, skill_manage, cronjob).
• Adds pstack:deslop (ported from cursor-team-kit) and links it in the guide.
• Restores correct attribution (originally a Cursor plugin by poteto, adapted for Hermes).

So it never happens again

• __init__.py auto-registers every skills/*/SKILL.md (no hand-maintained list).
• scripts/check_refs.py fails on: broken relative links, any pstack:<name> that has no skill (code spans included), missing playbooks, bad frontmatter, em dashes, and Cursor leftovers (.cursor/, AskQuestion, Task tool, transcripts paths...). No allowlist.
• .github/workflows/check.yml runs it on every push and PR. AGENTS.md makes it mandatory before commit.

Verified locally: checker exit 0 on the tree; exit 1 when injecting a fake pstack: ref, a broken link, or a ~/.cursor path; loader registers 48 skills incl. deslop and setup-pstack.