[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/lib.rs

@@ -12,6 +12,7 @@ pub mod scoring;
 pub mod entity;
 pub mod edge;
 pub mod community;
+pub mod agent_entity;
 
 pub use gate_parser::{GateResponse, ParseError, parse_gate_response};
 
@@ -30,3 +31,4 @@ pub use scoring::{DocumentScorer, ScoringPipeline, GlobalTfIdfScorer, ProjectTfI
 pub use entity::{Entity, EntityType};
 pub use edge::{Edge, ContradictionStatus};
 pub use community::Community;
+pub use agent_entity::{AgentPromptMeta, AgentSkillMeta, AgentDecisionMeta, DecisionOutcome};