refactor: sqlc vs raw SQL — hybrid approach (R3)

Deleted 8 genuinely unused sqlc queries (no inline equivalent):
- UpsertCurrentRelationship, ListEntitiesCapped, ListEntityStatus,
  UpdateSignalState, InsertClassification, InsertFeedback, InsertSkill,
  UpsertCurrentRelationship — all had zero call sites.

Migrated 9 inline raw SQL sites to use sqlc queries:
- GetOntology (impl.go): ListEntityTypes, ListRelationshipTypes,
  ListLifecycleDefs — replaces 3 raw pool.Query blocks with typed sqlcgen
  calls, eliminating manual row scanning.
- EndRelationship (phase3.go): EndCurrentRelationship — replaces tx.Exec
  with sqlcgen.New(tx).EndCurrentRelationship.
- checkPrecondition (impl.go): GetEntityStatus — replaces tx.QueryRow +
  manual Scan with sqlcgen.New(tx).GetEntityStatus.
- GetEntityRelations (impl.go): ListEntityRelations — replaces raw pool.Query
  + scanRelationships helper (now deleted).
- GetGraph (impl.go): ListGraphEdges — replaces raw pool.Query +
  scanRelationships.
- resolveEntityID (impl.go): GetEntityBySlug/GetEntityByID — replaces
  raw pool.QueryRow + Scan.
- createApproval (mcp/server.go): InsertApproval — replaces raw pool.Exec
  with sqlcgen.InsertApproval.

Deleted scanRelationships helper (was only used by the two migrated
graph queries above).

Regenerated sqlcgen — also picks up stale model updates (AgentSession,
SessionPlanStep, SessionQuestion, etc. from recent migrations).

Documented the carve-out in .agents/dev/CONTRIBUTING.md §SQL conventions:
sqlc is the default; raw pool.Query/Exec is reserved for LISTEN/NOTIFY,
dynamic WHERE builders, blast_radius(), and COPY.

go vet, build, httpapi/mcp/db tests all pass. -383/+170 lines.

internal/httpapi/phase3.go

@@ -2177,15 +2177,15 @@ func (s *Server) EndRelationship(ctx context.Context, req gen.EndRelationshipReq
 	}
 	defer tx.Rollback(ctx)
 
-	result, err := tx.Exec(ctx, `
-		UPDATE relationships
-		SET valid_to = now()
-		WHERE source_id = $1 AND target_id = $2 AND type = $3 AND valid_to IS NULL`,
-		sourceID, targetID, req.Params.RelType)
+	result, err := sqlcgen.New(tx).EndCurrentRelationship(ctx, sqlcgen.EndCurrentRelationshipParams{
+		SourceID: sourceID,
+		TargetID: targetID,
+		Type:     req.Params.RelType,
+	})
 	if err != nil {
 		return nil, err
 	}
-	if result.RowsAffected() == 0 {
+	if result == 0 {
 		return nil, fmt.Errorf("%w: active relationship %s:%s:%s",
 			domain.ErrNotFound, req.Params.Source, req.Params.RelType, req.Params.Target)
 	}