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/advanced/20-patterns.md

@@ -277,12 +277,12 @@ schedule BaseWorker {
     block lunch { 12:00 - 13:00, action: social::lunch }
 }
 
-schedule EarlyBird extends BaseWorker {
+schedule EarlyBird modifies BaseWorker {
     block work { 05:00 - 13:00, action: work::early_shift }
     block lunch { 11:00 - 12:00, action: social::lunch }
 }
 
-schedule NightOwl extends BaseWorker {
+schedule NightOwl modifies BaseWorker {
     block work { 14:00 - 22:00, action: work::late_shift }
     block lunch { 18:00 - 19:00, action: social::dinner }
 }