fix(repo,service): SCALE-002 — push pagination into SQL for target/issuer/team/agent_group

Sprint 2 unified-master-audit closure. Pre-fix four service List
endpoints (target, issuer, team, agent_group) called repoFoo.List(ctx)
to fetch the full table then sliced in memory:

    rows, _ := s.repo.List(ctx)
    total := int64(len(rows))
    start := (page - 1) * perPage
    end := start + perPage
    return rows[start:end], total, nil

This page-sliced in memory pattern marshals every row per request —
fine on small fleets but unacceptable for multi-tenant or large-fleet
deploys. The agent_group case was worse — the service explicitly
ignored page/perPage and returned the entire slice.

Fix:
  - New ListPaginated(ctx, limit, offset) method on each of the four
    repositories. Postgres implementations push LIMIT + OFFSET into
    the SQL plus a SELECT COUNT(*) for the total. Mirrors the cursor
    pattern already in internal/repository/postgres/certificate.go.
  - Each ListPaginated normalises limit≤0→50 and offset<0→0,
    matching the service-layer defaults that already existed.
  - Repository interfaces grow the new method so adapters stay
    swappable.
  - Service List methods now call repoFoo.ListPaginated(ctx, perPage,
    (page-1)*perPage) directly — no more memory-slice.
  - AgentGroupService.ListAgentGroups closes the Bundle E / Audit
    L-020 'page/perPage unused' gap.

Test changes:
  - sliceWindow generic helper in testutil_test.go mirrors the SQL
    LIMIT/OFFSET semantics for in-memory mocks.
  - Six mock implementers (lifecycle_test, testutil_test x2,
    agent_group_test, team_test) gain ListPaginated methods.
  - TestTeamService_List_SCALE002_PaginationPropagatesToRepo pins
    the page=2, perPage=3 → 3 rows of 10 invariant.

Closes SCALE-002.

internal/repository/postgres/agent_group.go

@@ -44,6 +44,40 @@ func (r *AgentGroupRepository) List(ctx context.Context) ([]*domain.AgentGroup,
 	return groups, rows.Err()
 }
 
+// ListPaginated returns a slice of agent groups bounded by limit/offset
+// plus the total count. SCALE-002 closure (Sprint 2, 2026-05-16).
+func (r *AgentGroupRepository) ListPaginated(ctx context.Context, limit, offset int) ([]*domain.AgentGroup, int64, error) {
+	if limit <= 0 {
+		limit = 50
+	}
+	if offset < 0 {
+		offset = 0
+	}
+	var total int64
+	if err := r.db.QueryRowContext(ctx, `SELECT COUNT(*) FROM agent_groups`).Scan(&total); err != nil {
+		return nil, 0, fmt.Errorf("failed to count agent groups: %w", err)
+	}
+	rows, err := r.db.QueryContext(ctx,
+		`SELECT id, name, description, match_os, match_architecture, match_ip_cidr, match_version, enabled, created_at, updated_at
+		 FROM agent_groups ORDER BY name LIMIT $1 OFFSET $2`, limit, offset)
+	if err != nil {
+		return nil, 0, fmt.Errorf("failed to query agent groups: %w", err)
+	}
+	defer rows.Close()
+	var groups []*domain.AgentGroup
+	for rows.Next() {
+		g, err := scanAgentGroup(rows)
+		if err != nil {
+			return nil, 0, err
+		}
+		groups = append(groups, g)
+	}
+	if err := rows.Err(); err != nil {
+		return nil, 0, err
+	}
+	return groups, total, nil
+}
+
 // Get retrieves an agent group by ID.
 func (r *AgentGroupRepository) Get(ctx context.Context, id string) (*domain.AgentGroup, error) {
 	row := r.db.QueryRowContext(ctx,