feat: Simplify phase deliverables and add 'execute' contract type

## Changes

### Phase Deliverables Simplified
- **Simple contract type**:
  - Plan phase: Only 'Plan' deliverable (required)
  - Execute phase: Only 'PR' deliverable (required)

- **Specification contract type**:
  - Research phase: Only 'Research Notes' deliverable (required)
  - Specify phase: Only 'Requirements Document' deliverable (required)
  - Plan phase: Only 'Plan' deliverable (required)
  - Execute phase: Only 'PR' deliverable (required)
  - Review phase: Only 'Release Notes' deliverable (required)

### New 'execute' Contract Type
- Only has 'execute' phase (no plan or review phases)
- NO deliverables at all - executes tasks directly
- Added to ContractType enum with proper Display/FromStr implementations
- Added helper methods: `initial_phase()`, `terminal_phase()`

### API Updates
- Added `get_phase_deliverables_for_type()` for contract-type-aware deliverables
- Added `get_phase_checklist_for_type()` for contract-type-aware checklists
- Added `check_phase_completion_for_type()` for contract-type-aware completion checks
- Legacy functions remain for backward compatibility (default to 'simple' type)

### Files Modified
- makima/src/llm/phase_guidance.rs - Core deliverable definitions
- makima/src/db/models.rs - ContractType enum and Contract methods
- makima/src/llm/mod.rs - Export new functions
- makima/src/server/handlers/contract_daemon.rs - Use type-aware functions
- makima/src/server/handlers/contract_chat.rs - Use type-aware functions

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

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