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/implement/SKILL.md

@@ -16,9 +16,6 @@ code quality) before moving on. Subagents have isolated context; the
 orchestrator only sees their reports. This is what gives AILang's
 development cycle its context budget back.
 
-The pattern is `superpowers:subagent-driven-development` adapted to
-the AILang agent toolchain.
-
 ## When to Use / Skipping
 
 Triggers:
@@ -181,10 +178,6 @@ Reviewer subagents typically need at least the standard model.
 
 ## Cross-references
 
-- **Upstream pattern:** `superpowers:subagent-driven-development` —
-  source of the per-task fresh-subagent + two-stage review pattern.
-  `superpowers:executing-plans` for the parallel-session variant
-  (rarely used in AILang).
 - **Agents dispatched:**
   - `skills/implement/agents/ailang-implementer.md` — task execution
   - `skills/implement/agents/ailang-tester.md` — E2E coverage