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/parser.lalrpop

@@ -386,16 +386,16 @@ Schedule: Schedule = {
     // Simple schedule: schedule Name { ... }
     <start:@L> "schedule" <name:Ident> "{" <body:ScheduleBody> "}" <end:@R> => Schedule {
         name,
-        extends: None,
+        modifies: None,
         fields: body.0,
         blocks: body.1,
         recurrences: body.2,
         span: Span::new(start, end),
     },
-    // Extending schedule: schedule Name extends Base { ... }
-    <start:@L> "schedule" <name:Ident> "extends" <base:Ident> "{" <body:ScheduleBody> "}" <end:@R> => Schedule {
+    // Modifying schedule: schedule Name modifies Base { ... }
+    <start:@L> "schedule" <name:Ident> "modifies" <base:Ident> "{" <body:ScheduleBody> "}" <end:@R> => Schedule {
         name,
-        extends: Some(base),
+        modifies: Some(base),
         fields: body.0,
         blocks: body.1,
         recurrences: body.2,
@@ -1003,7 +1003,7 @@ extern {
         "schedules" => Token::Schedules,
         "tree" => Token::Tree,
         "priority" => Token::Priority,
-        "extends" => Token::Extends,
+        "modifies" => Token::Modifies,
         "override" => Token::Override,
         "recurrence" => Token::Recurrence,
         "season" => Token::Season,