refactor: drop superpowers references in SKILL.md, add .claude/skills/ discovery

The skills are functionally complete and standalone — superpowers
references were attribution-only. Removed so a reader without the
superpowers plugin loaded isn't sent chasing dead links.

Plus: .claude/skills/<name> symlinks so Claude Code's Skill tool can
invoke them by name (analogous to the existing .claude/agents/
symlinks). Discovery section in skills/README.md updated to cover
both symlink sets.

skills/debug/SKILL.md

@@ -36,9 +36,9 @@ NO FIXES WITHOUT ROOT CAUSE INVESTIGATION FIRST
 NO FIX WITHOUT A FAILING TEST FIRST
 ```
 
-Both clauses are non-negotiable. The first is from
-`superpowers:systematic-debugging`. The second is the project's
-own rule (formerly in CLAUDE.md "Bug fixes — TDD, always").
+Both clauses are non-negotiable. The second was formerly the
+CLAUDE.md "Bug fixes — TDD, always" section before the
+2026-05-09 skill-system migration.
 
 ## The Process — four phases
 
@@ -143,8 +143,6 @@ All of these mean: **return to Phase 1**.
 
 ## Cross-references
 
-- **Upstream pattern:** `superpowers:systematic-debugging` —
-  source of the four-phase Iron Law and Phase 4.5.
 - **Agent dispatched:** `skills/debug/agents/ailang-debugger.md` —
   carries the actual diagnostic work (read CLAUDE.md / DESIGN.md /
   JOURNAL, reproduce, write RED test, propose minimal fix).