feat(skills): add just-commands skill with dynamic context injection

Add a new skill that uses Claude Code's dynamic context injection feature
to embed the output of `just --list` at runtime. This replaces the
static command list in CLAUDE.md with a reference to the skill.

Benefits:
- Commands stay in sync with justfile automatically
- Reduces manual maintenance of CLAUDE.md
- Demonstrates Claude Code's !`command` syntax for dynamic context

Author: ryoppippi

.claude/skills/just-commands/SKILL.md

@@ -0,0 +1,11 @@
+---
+name: just-commands
+description: Available just commands for this project. Use when you need to know what development commands are available.
+user-invocable: false
+---
+
+## Available just commands
+
+```
+!`just --list`
+```

CLAUDE.md

@@ -27,20 +27,7 @@ StackOne AI SDK is a Python library that provides a unified interface for access
 
 ## Essential Development Commands
 
-```bash
-# Setup and installation
-just install           # Install dependencies and pre-commit hooks
-
-# Code quality
-just lint             # Run ruff linting
-just lint-fix         # Auto-fix linting issues
-just ty               # Run type checking
-
-# Testing
-just test             # Run all tests
-just test-tools       # Run tool-specific tests
-just test-examples    # Run example tests
-```
+Run `just --list` to see all available commands, or Claude will automatically reference the `just-commands` skill when needed.
 
 ## Code Architecture