docs: replace AGENTS.md skill list with discovery instructions

A static skill list in AGENTS.md goes stale as new skills are added
(it already missed the make-pythonic skill that was merged separately).
Replace the enumerated list with a pointer telling agents to list
.ai/skills/ and read each SKILL.md frontmatter, so the catalog never
has to be hand-maintained.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: timsaucer

AGENTS.md

@@ -33,11 +33,9 @@ Skills follow the [Agent Skills](https://agentskills.io) open standard. Each ski
 - `SKILL.md` — The skill definition with YAML frontmatter (name, description, argument-hint) and detailed instructions.
 - Additional supporting files as needed.
 
-Currently available skills:
-
-- [`check-upstream`](.ai/skills/check-upstream/SKILL.md) — audit upstream
-  Apache DataFusion features (functions, DataFrame ops, SessionContext
-  methods, FFI types) not yet exposed in the Python bindings.
+To discover what skills are available, list `.ai/skills/` and read each
+`SKILL.md`. The frontmatter `name` and `description` fields summarize the
+skill's purpose.
 
 ## Pull Requests