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.

tree-sitter-storybook/package.json

@@ -1,6 +1,6 @@
 {
   "name": "tree-sitter-storybook",
-  "version": "0.1.0",
+  "version": "0.3.1",
   "description": "Tree-sitter grammar for Storybook narrative DSL",
   "main": "bindings/node",
   "types": "bindings/node",