Make README upstream coverage section tool-agnostic

Remove Claude Code references and update skill path from .claude/skills/
to .ai/skills/ to match the new tool-neutral directory structure.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: timsaucer

README.md

@@ -314,30 +314,30 @@ There are scripts in `ci/scripts` for running Rust and Python linters.
 
 ## Checking Upstream DataFusion Coverage
 
-This project includes a [Claude Code](https://claude.com/claude-code) skill for auditing which
+This project includes an [AI agent skill](.ai/skills/check-upstream/SKILL.md) for auditing which
 features from the upstream Apache DataFusion Rust library are not yet exposed in these Python
 bindings. This is useful when adding missing functions, auditing API coverage, or ensuring parity
 with upstream.
 
-To use it, run the `/check-upstream` slash command inside Claude Code with an optional area argument:
+The skill accepts an optional area argument:
 
 ```
-/check-upstream scalar functions
-/check-upstream aggregate functions
-/check-upstream window functions
-/check-upstream dataframe
-/check-upstream session context
-/check-upstream all
+scalar functions
+aggregate functions
+window functions
+dataframe
+session context
+ffi types
+all
 ```
 
 If no argument is provided, it defaults to checking all areas. The skill will fetch the upstream
 DataFusion documentation, compare it against the functions and methods exposed in this project, and
 produce a coverage report listing what is currently exposed and what is missing.
 
-The skill definition lives in `.claude/skills/check-upstream/SKILL.md`. Note that the `/check-upstream`
-slash command is a [Claude Code](https://claude.com/claude-code) feature, but the underlying
-methodology described in the skill file can be followed manually or by another AI coding agent if
-directed to read and follow the instructions in that file.
+The skill definition lives in `.ai/skills/check-upstream/SKILL.md` and follows the
+[Agent Skills](https://agentskills.io) open standard. It can be used by any AI coding agent that
+supports skill discovery, or followed manually.
 
 ## How to update dependencies