feat(lang): complete extends to modifies keyword migration

This commit completes the migration started in the previous commit,
updating all remaining files:

- Lexer: Changed token from Extends to Modifies
- Parser: Updated lalrpop grammar rules and AST field names
- AST: Renamed Schedule.extends field to modifies
- Grammar: Updated tree-sitter grammar.js
- Tree-sitter: Regenerated parser.c and node-types.json
- Examples: Updated baker-family work schedules
- Tests: Updated schedule composition tests and corpus
- Docs: Updated all reference documentation and tutorials
- Validation: Updated error messages and validation logic
- Package: Bumped version to 0.3.1 in all package manifests

All 554 tests pass.

.claude/CLAUDE.md

@@ -12,6 +12,20 @@ These rules are **MANDATORY** for all commits:
 
 ## Development Workflow
 
+### Cleaning Build Artifacts
+
+To clean all build artifacts across the entire workspace:
+
+```bash
+cargo xtask clean
+```
+
+This removes:
+- Rust build artifacts (`target/`, `Cargo.lock`)
+- tree-sitter node_modules and build output
+- Zed extension build artifacts (`grammars/`, `extension.wasm`)
+- mdbook build artifacts (`docs/book/`)
+
 ### Installing the LSP
 
 To reinstall/update the Storybook LSP after making changes: