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.

docs/SBIR-v0.2.0-SPEC.md

@@ -936,7 +936,7 @@ Character:
 
 **Source:**
 ```storybook
-schedule WorkWeek extends BaseSchedule {
+schedule WorkWeek modifies BaseSchedule {
     block morning { 08:00 - 12:00: WorkTasks }
     block lunch { 12:00 - 13:00: EatLunch }
     block afternoon { 13:00 - 17:00: WorkTasks }