feat(phase-3.1): agent entity types + metadata structs

EntityType enum extended with 3 agent types:
  - AgentPrompt: track prompt templates, usage, quality
  - AgentSkill: track learned capabilities, success rate, latency
  - AgentDecision: track decisions, reasoning, outcomes

New module: agent_entity.rs (280 LOC)
  Structs: AgentPromptMeta, AgentSkillMeta, AgentDecisionMeta, DecisionOutcome
  Factories: new_agent_prompt(), new_agent_skill(), new_agent_decision()
  Updaters: record_prompt_usage(), record_skill_invocation(), record_decision_outcome()
  Exports: added to mem-core lib.rs

Tests: 8 new (prompt, skill, decision, outcome, usage stats,
       invocation stats, round-trip, serialization)
Build: cargo build --release clean
Suite: 174 lib tests pass

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);