Move user guide from python/datafusion/AGENTS.md to SKILL.md

The in-wheel AGENTS.md was not a real distribution channel -- no shipping
agent walks site-packages for AGENTS.md files. Moving to SKILL.md at the
repo root, with YAML frontmatter, lets the skill ecosystems (npx skills,
Claude Code plugin marketplaces, community aggregators) discover it.

Update the pointers in the contributor AGENTS.md and the __init__.py
module docstring accordingly. The docstring now references the GitHub
URL since the file no longer ships with the wheel.

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

Author: timsaucer

AGENTS.md

@@ -22,7 +22,7 @@
 This file is for agents working **on** the datafusion-python project (developing,
 testing, reviewing). If you need to **use** the DataFusion DataFrame API (write
 queries, build expressions, understand available functions), see the user-facing
-guide at [`python/datafusion/AGENTS.md`](python/datafusion/AGENTS.md).
+skill at [`SKILL.md`](SKILL.md).
 
 ## Skills
 

python/datafusion/AGENTS.md SKILL.mdpython/datafusion/AGENTS.md renamed to SKILL.md

@@ -17,6 +17,11 @@
   under the License.
 -->
 
+---
+name: datafusion-python
+description: Use when the user is writing datafusion-python (Apache DataFusion Python bindings) DataFrame or SQL code. Covers imports, data loading, DataFrame operations, expression building, SQL-to-DataFrame mappings, idiomatic patterns, and common pitfalls.
+---
+
 # DataFusion Python DataFrame API Guide
 
 ## What Is DataFusion?

python/datafusion/__init__.py

@@ -51,7 +51,8 @@
 
 For a comprehensive guide to the DataFrame API -- including a SQL-to-DataFrame
 reference table, expression building, idiomatic patterns, and common pitfalls --
-see the ``AGENTS.md`` file shipped alongside this package.
+see ``SKILL.md`` at the root of the datafusion-python repository:
+https://github.com/apache/datafusion-python/blob/main/SKILL.md
 
 Full documentation: https://datafusion.apache.org/python
 """