feat: Phase 6 — KnowledgeService + LearningService + postgres repos

Problem: knowledge and learning operations were scattered across
httpapi and mcp handlers with no shared service layer. The hexagonal
refactor needs a single use-case service for both surfaces.

Change:
- app/knowledge.go: KnowledgeService (Search, Upsert, GetContent,
  SoftDelete, Restore) and LearningService (ListPatterns,
  UpsertPattern, Validate, Quarantine) wrapping the port interfaces.
- adapters/postgres/knowledge.go: KnowledgeRepo implements
  KnowledgeRepository — Search, GetContent, Upsert, SoftDelete,
  Restore with inline SQL matching the existing handler patterns
  (full-text search ILIKE, upsert on conflict, soft-delete).

Verification: go build/vet, full test suite (18 pkgs), DB integration
(postgres + mcp — green).

internal/core/app/knowledge.go

@@ -0,0 +1,91 @@
+package app
+
+import (
+	"context"
+	"time"
+
+	"github.com/dtoro/oikos/internal/core/domain"
+	"github.com/dtoro/oikos/internal/core/ports"
+)
+
+// KnowledgeService is the use-case layer for knowledge-base operations.
+// Shared by REST + MCP knowledge tools (Phase 6 convergence).
+type KnowledgeService struct {
+	repo ports.KnowledgeRepository
+}
+
+func NewKnowledgeService(repo ports.KnowledgeRepository) *KnowledgeService {
+	return &KnowledgeService{repo: repo}
+}
+
+type SearchCmd struct {
+	Query string
+	Limit int
+}
+
+func (s *KnowledgeService) Search(ctx context.Context, cmd SearchCmd) ([]ports.KnowledgeEntry, error) {
+	return s.repo.Search(ctx, cmd.Query, cmd.Limit)
+}
+
+type UpsertKnowledgeCmd struct {
+	Entry  KnowledgeEntryCmd
+	ActorType string
+	Actor     string
+}
+
+type KnowledgeEntryCmd struct {
+	Slug    string
+	Title   string
+	Kind    string
+	Tags    []string
+	Content string
+	About   []string
+}
+
+func (s *KnowledgeService) Upsert(ctx context.Context, cmd UpsertKnowledgeCmd) (ports.KnowledgeEntry, error) {
+	return s.repo.Upsert(ctx, ports.KnowledgeUpsertInput{
+		Entry: ports.KnowledgeEntry{
+			Slug: cmd.Entry.Slug, Title: cmd.Entry.Title,
+			Kind: cmd.Entry.Kind, Tags: cmd.Entry.Tags,
+			Content: cmd.Entry.Content, About: cmd.Entry.About,
+			UpdatedAt: time.Now(),
+		},
+	})
+}
+
+func (s *KnowledgeService) GetContent(ctx context.Context, slug string) (ports.KnowledgeEntry, error) {
+	return s.repo.GetContent(ctx, slug)
+}
+
+func (s *KnowledgeService) SoftDelete(ctx context.Context, slug string) error {
+	return s.repo.SoftDelete(ctx, slug)
+}
+
+func (s *KnowledgeService) Restore(ctx context.Context, slug string) error {
+	return s.repo.Restore(ctx, slug)
+}
+
+// LearningService handles patterns, feedback, and skills.
+type LearningService struct {
+	repo ports.LearningRepository
+}
+
+func NewLearningService(repo ports.LearningRepository) *LearningService {
+	return &LearningService{repo: repo}
+}
+
+func (s *LearningService) ListPatterns(ctx context.Context) ([]domain.Pattern, error) {
+	return s.repo.ListPatterns(ctx, 100)
+}
+
+func (s *LearningService) UpsertPattern(ctx context.Context, pattern domain.Pattern) error {
+	return s.repo.UpsertPattern(ctx, pattern)
+}
+
+func (s *LearningService) Validate(ctx context.Context, patternID domain.UUID) error {
+	return s.repo.Validate(ctx, patternID)
+}
+
+func (s *LearningService) Quarantine(ctx context.Context, patternID domain.UUID, reason string) error {
+	return s.repo.Quarantine(ctx, patternID, reason)
+}