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.

src/syntax/lexer.rs

@@ -87,8 +87,8 @@ pub enum Token {
     Tree,
     #[token("priority")]
     Priority,
-    #[token("extends")]
-    Extends,
+    #[token("modifies")]
+    Modifies,
     #[token("override")]
     Override,
     #[token("recurrence")]