feat: add tool execution idempotency cache

For tools that declare `is_idempotent() == true`, successful results are
cached per-job so repeated identical calls (common during self-repair
recovery, stuck job retries, or chat-mode retry loops) return instantly
without re-executing. Cache uses SHA-256 content hashing with TTL and
LRU eviction, scoped per job_id.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

src/testing.rs

@@ -282,6 +282,9 @@ impl TestHarnessBuilder {
             max_actions_per_hour: None,
         }));
 
+        let idempotency_cache = Arc::new(crate::tools::ToolIdempotencyCache::new(
+            crate::tools::IdempotencyCacheConfig::default(),
+        ));
         let deps = AgentDeps {
             store: Some(Arc::clone(&db)),
             llm,
@@ -294,6 +297,7 @@ impl TestHarnessBuilder {
             skills_config: SkillsConfig::default(),
             hooks,
             cost_guard,
+            idempotency_cache,
         };
 
         TestHarness {