feat(T2.3): implement prompt template caching engine

- Add internal/templates package for Go template pre-compilation
- Implement TemplateEngine with compile-once-render-many pattern
- Template caching with LRU eviction policy
- Configurable max cache size (default 100)
- Compile-time tracking for performance analysis
- Per-template render count and latency metrics
- Cache statistics: hit ratio, avg render time, total renders
- CompileAndRender() for single-call compile+render
- Thread-safe concurrent access with RWMutex
- 17 template tests, all passing

Features:
- Compile() caches compiled templates
- Render() uses cached templates for fast rendering
- GetStats() tracks per-template metrics
- GetCacheStats() shows overall cache health
- Clear() resets all cached templates
- Remove() removes specific template
- IsCached() checks if template is pre-compiled

Performance:
- Template render latency: <100ms ✓
- Caching eliminates parse overhead
- LRU eviction when cache full
- Concurrent render support
- Compile once, render many times

Verification:
- Render latency < 100ms (verified in tests)
- Cache eviction working correctly
- Stats tracking accurate
- Complex templates supported
- Error handling robust

Test Coverage:
- 17 template tests (compile, render, caching, stats)
- Latency verification (< 100ms)
- Complex template support
- LRU eviction testing
- Concurrent access patterns

Next: T2.4 (Lessons file indexing)

tasks/board-T2.md

@@ -6,7 +6,7 @@
 |----|-------|--------|--------|--------------|
 | T2.1 | Activity result caching: deduplicate repeated LLM calls for same task state | [x] | `task/T2.1` | Implementer called 2x on same code → second call returns cached Implementer output |
 | T2.2 | Parallel task dispatch: multiple T0.x tasks execute truly concurrently (not sequential) | [x] | `task/T2.2` | 9 tasks complete in ~1/9 total time (wall-clock speedup measured) |
-| T2.3 | Prompt template caching: pre-compile Go templates on worker startup | [ ] | `task/T2.3` | Template render latency < 100ms (vs parse+render each time) |
+| T2.3 | Prompt template caching: pre-compile Go templates on worker startup | [x] | `task/T2.3` | Template render latency < 100ms (vs parse+render each time) |
 | T2.4 | Lessons file indexing: fast lookup of past failures without full file scan | [ ] | `task/T2.4` | Query lessons by task type → return in < 10ms for 1000s of entries |
 | T2.5 | Git operation batching: combine multiple worktree commits into single push/merge | [ ] | `task/T2.5` | N tasks → 1 push (vs N pushes), measured via git ref-log |
 | T2.6 | LLM request batching: group similar Implementer calls into one API request | [ ] | `task/T2.6` | 3 implementer tasks → 1 Anthropic API call with batch input (vs 3 separate calls) |