[Phase 3.1] Agent entity types + metadata structs (#47)

## Changes
- `crates/mem-core/src/entity.rs` — Added AgentPrompt, AgentSkill, AgentDecision to EntityType enum
- `crates/mem-core/src/agent_entity.rs` — New module (280 LOC): metadata structs, factories, stat updaters
- `crates/mem-core/src/lib.rs` — Module registration + exports

## Agent Entity Types
- **AgentPrompt**: template, target_model, task_category, usage_count, avg_quality, version
- **AgentSkill**: description, trigger_patterns, success_rate, invocation_count, avg_latency_ms
- **AgentDecision**: action, reasoning, alternatives, confidence, outcome (success/quality/feedback)

## Validation
- 8 new tests pass (factories, stats, round-trip, serialization)
- 174 total lib tests pass
- `cargo build --release` cleanReviewed-on: #47

Co-authored-by: rock <[email protected]>

crates/mem-core/src/entity.rs

@@ -17,6 +17,13 @@ pub enum EntityType {
     Location,
     Event,
     Organization,
+    /// Agent prompt template tracked as a first-class entity.
+    /// Enables the agent to learn which prompts produce good results.
+    AgentPrompt,
+    /// Agent skill — a reusable capability the agent has learned.
+    AgentSkill,
+    /// Agent decision — a recorded choice with reasoning and outcome.
+    AgentDecision,
     Unknown,
 }
 
@@ -29,6 +36,9 @@ impl EntityType {
             Self::Location => "location",
             Self::Event => "event",
             Self::Organization => "organization",
+            Self::AgentPrompt => "agent_prompt",
+            Self::AgentSkill => "agent_skill",
+            Self::AgentDecision => "agent_decision",
             Self::Unknown => "unknown",
         }
     }
@@ -41,6 +51,9 @@ impl EntityType {
             "location" => Self::Location,
             "event" => Self::Event,
             "organization" => Self::Organization,
+            "agent_prompt" => Self::AgentPrompt,
+            "agent_skill" => Self::AgentSkill,
+            "agent_decision" => Self::AgentDecision,
             _ => Self::Unknown,
         }
     }
@@ -175,6 +188,9 @@ mod tests {
             EntityType::Person,
             EntityType::Tool,
             EntityType::Concept,
+            EntityType::AgentPrompt,
+            EntityType::AgentSkill,
+            EntityType::AgentDecision,
         ] {
             let s = ty.as_str();
             assert_eq!(EntityType::from_str(s), *ty);