feat(prompts): implement Phase 4 - Execution Engine (FR-4, FR-5)

Implement three-stage execution lifecycle with idempotent runs and complete
provenance tracking via RunManifest.

Core Features:
- PromptRun model with execution lifecycle stages:
  1. Analysis: Template analysis and macro extraction
  2. Compilation: Macro resolution and context compilation
  3. Processing: LLM execution and output generation
- InputBundleHash for deterministic idempotency (FR-4.3)
- RunManifest for complete execution provenance (FR-5)
- LLMAdapter interface for pluggable model providers
- MockLLMAdapter for testing without API calls
- PromptExecutionEngine orchestrating full lifecycle

Idempotent Execution (FR-4.4):
- Calculate SHA-256 hash of complete input context
- Skip execution if identical hash exists
- Cache successful runs by hash
- Support force re-execution via config flag

RunManifest Tracking (FR-5.2):
- Template metadata (id, name, digest)
- Resolved input artifacts and digests
- Compiled prompt digest
- Model configuration
- Output artifacts
- Dependency edges for graph construction
- Timing metadata for performance analysis

Tests (27 passing):
- 17 execution model tests (config, bundle, runs, stages)
- 10 engine tests (execution, idempotency, errors, caching)

Implements:
- FR-4.1: Three-stage execution lifecycle
- FR-4.2: CompiledPrompt during compilation
- FR-4.3: InputBundleHash calculation
- FR-4.4: Skip execution for identical hashes
- FR-5.1: RunManifest persistence
- FR-5.2: Complete manifest contents
- FR-5.3: Nested run linking (foundation)

Files Created:
- markitect/prompts/execution/models.py
- markitect/prompts/execution/manifest.py
- markitect/prompts/execution/llm_adapter.py
- markitect/prompts/execution/engine.py
- migrations/prompts/003_create_runs_and_manifests.sql
- tests/unit/prompts/test_execution_models.py
- tests/unit/prompts/test_execution_engine.py

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

markitect/prompts/execution/__init__.py

@@ -0,0 +1,29 @@
+"""
+Execution engine for Prompt Dependency Resolution.
+
+This package provides the core execution infrastructure for running
+PromptTemplates with idempotent execution and complete provenance tracking.
+"""
+
+from markitect.prompts.execution.models import (
+    PromptRun,
+    ExecutionStage,
+    RunConfig,
+    InputBundle,
+    LLMResponse,
+)
+from markitect.prompts.execution.manifest import RunManifest
+from markitect.prompts.execution.engine import PromptExecutionEngine
+from markitect.prompts.execution.llm_adapter import LLMAdapter, MockLLMAdapter
+
+__all__ = [
+    "PromptRun",
+    "ExecutionStage",
+    "RunConfig",
+    "InputBundle",
+    "LLMResponse",
+    "RunManifest",
+    "PromptExecutionEngine",
+    "LLMAdapter",
+    "MockLLMAdapter",
+]