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/reference/19-validation.md

@@ -163,8 +163,8 @@ life_arc HasOrphan {
 | Rule | Description | Severity |
 |------|-------------|----------|
 | Time format | Times must be valid HH:MM or HH:MM:SS | Error |
-| Extends exists | Base schedule must be defined | Error |
-| No circular extends | Schedule chains cannot form cycles | Error |
+| Modifies exists | Base schedule must be defined | Error |
+| No circular modifies | Schedule chains cannot form cycles | Error |
 | Named blocks unique | Block names must be unique within a schedule | Error |
 | Action references valid | Action references must resolve to defined behaviors | Error |
 | Constraint values valid | Temporal constraint values must reference defined enums | Error |
@@ -182,10 +182,10 @@ schedule Bad {
 }
 ```
 
-**Circular extends:**
+**Circular modifies:**
 ```storybook
-schedule A extends B { }
-schedule B extends A { }  // Error: circular schedule extension detected
+schedule A modifies B { }
+schedule B modifies A { }  // Error: circular schedule extension detected
 ```
 
 ## Relationship Validation
@@ -359,7 +359,7 @@ character Good {
 | Template | Unique name, valid includes, valid ranges, strict enforcement |
 | Behavior | Non-empty, valid composites, valid decorators, valid subtrees |
 | Life Arc | Non-empty, unique states, valid transitions, reachable states |
-| Schedule | Valid times, valid extends chain, unique block names |
+| Schedule | Valid times, valid modifies chain, unique block names |
 | Relationship | >= 2 participants, valid references |
 | Species | Unique name, valid includes, no cycles |
 | Enum | Unique name, unique variants, non-empty |