[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]>

.gitea/workflows/build.yaml

@@ -50,19 +50,13 @@ jobs:
           REGISTRY_USER: ${{ secrets.FORGEJO_REGISTRY_USER }}
           REGISTRY_TOKEN: ${{ secrets.FORGEJO_REGISTRY_TOKEN }}
 
-      - name: Build Docker image
+      - name: Build and push Docker image (SHA tag only)
         run: |
           docker build --no-cache --progress=plain \
             -t "${IMAGE}:${{ steps.sha.outputs.short_sha }}" \
-            -t "${IMAGE}:latest" \
             -f Dockerfile .
-
-      - name: Push Docker image
-        if: github.event_name == 'push' || github.event_name == 'workflow_dispatch'
-        run: |
           docker push "${IMAGE}:${{ steps.sha.outputs.short_sha }}"
-          docker push "${IMAGE}:latest"
-          echo "✓ Pushed: ${IMAGE}:${{ steps.sha.outputs.short_sha }}"
+          echo "Pushed: ${IMAGE}:${{ steps.sha.outputs.short_sha }}"
 
       - name: Prune unused images
         run: docker image prune -a --force 2>&1 | tail -3 || true