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.

examples/baker-family/README.md

@@ -52,7 +52,7 @@ Martha and Jane are a married couple who co-run the bakery. Emma is their daught
 - **Character inheritance**: Characters automatically inherit behaviors and schedules from templates
 
 ### Schedule Composition
-- **Schedule inheritance**: `BakerSchedule extends WorkWeek`
+- **Schedule inheritance**: `BakerSchedule modifies WorkWeek`
 - **Override blocks**: Modify inherited time blocks with `override work { ... }`
 - **Named blocks**: All blocks have names for the override system
 - **Action references**: Schedule blocks reference behavior trees via `action: BehaviorName`
@@ -132,7 +132,7 @@ WorkWeek
   ├─ work (09:00-17:00)
   └─ evening_rest (18:00-22:00)
 
-BakerSchedule extends WorkWeek
+BakerSchedule modifies WorkWeek
   ├─ pre_dawn_prep (04:00-05:00) [NEW]
   ├─ work (05:00-13:00) [OVERRIDE] -> action: BakingWork
   ├─ evening_rest (18:00-22:00) [INHERITED]
@@ -169,7 +169,7 @@ Maps baking skill tiers to quality expectations:
 1. **Martha (character)** -> inherits from **Baker (template)** -> inherits from **Human (species)**
    - Gets default fields from Human species (age, energy, mood, occupation)
    - Gets behaviors: `BakingSkills`, `CustomerService`, `BasicNeeds`, `SocialInteraction`
-   - Gets schedule: `BakerSchedule` (which extends `WorkWeek`)
+   - Gets schedule: `BakerSchedule` (which modifies `WorkWeek`)
    - Has `skill_tier: Master` for concept comparison mapping
 
 2. **Relationships** -> connect characters with asymmetric perspectives

examples/baker-family/schedules/work_schedules.sb

@@ -1,7 +1,7 @@
 //! Work schedules for the Baker family
 //!
 //! Demonstrates schedule composition features:
-//! - extends keyword for schedule inheritance
+//! - modifies keyword for schedule inheritance
 //! - override blocks to modify inherited schedules
 //! - Named blocks for override system
 //! - Action references to behavior trees
@@ -14,8 +14,8 @@ schedule WorkWeek {
     block evening_rest { 18:00 -> 22:00, action:Resting }
 }
 
-// Baker's schedule extends WorkWeek
-schedule BakerSchedule extends WorkWeek {
+// Baker's schedule modifies WorkWeek
+schedule BakerSchedule modifies WorkWeek {
     // Bakers start early - override the work block
     override work {
         05:00 -> 13:00
@@ -40,7 +40,7 @@ schedule BakerSchedule extends WorkWeek {
 }
 
 // Assistant baker schedule (helps during busy times)
-schedule AssistantSchedule extends BakerSchedule {
+schedule AssistantSchedule modifies BakerSchedule {
     // Assistant comes in later
     override work {
         06:00 -> 14:00