feat(P02): node management with SQLite-backed registry

Implements Phase 2 of v0.1 Foundation:
- internal/model/node.go: Node struct with state machine (pending/ready/left)
- internal/store/store.go: SQLite open with WAL + foreign_keys pragmas
- internal/store/migrate.go: embedded SQL migration runner
- internal/store/migrations/0001_nodes.sql: nodes table schema
- internal/store/node_repo.go: CRUD operations for nodes
- internal/store/node_repo_test.go: 4 tests covering insert/get/list/update/delete
- internal/engine/registry.go: in-memory wrapper with slog audit logging
- internal/cli/node.go: orca node {join,leave,list} wired to registry

Verified: node join/list/leave work end-to-end, JSON output, slog audit logs,
state persists in SQLite, all tests pass with -race.

---ci---
project: orca
phase: 2
milestone: v0.1
status: execute
req_covered:
  - REQ-002
  - REQ-005
  - REQ-008
  - REQ-012
  - REQ-017
  - REQ-018
---/ci---

internal/engine/registry.go

@@ -0,0 +1,57 @@
+package engine
+
+import (
+	"context"
+	"fmt"
+	"log/slog"
+
+	"git.cloudinit.dev/coreci/orca/internal/model"
+	"git.cloudinit.dev/coreci/orca/internal/store"
+)
+
+type NodeRegistry struct {
+	repo *store.NodeRepo
+	log  *slog.Logger
+}
+
+func NewNodeRegistry(repo *store.NodeRepo, log *slog.Logger) *NodeRegistry {
+	if log == nil {
+		log = slog.Default()
+	}
+	return &NodeRegistry{repo: repo, log: log}
+}
+
+func (r *NodeRegistry) Join(ctx context.Context, n *model.Node) error {
+	if err := r.repo.Insert(ctx, n); err != nil {
+		return fmt.Errorf("join node: %w", err)
+	}
+	r.log.Info("node joined",
+		slog.String("node_id", n.ID),
+		slog.String("name", n.Name),
+		slog.String("address", n.Address))
+	return nil
+}
+
+func (r *NodeRegistry) Leave(ctx context.Context, id string) error {
+	if err := r.repo.UpdateState(ctx, id, model.NodeStateLeft); err != nil {
+		return fmt.Errorf("leave node: %w", err)
+	}
+	r.log.Info("node left", slog.String("node_id", id))
+	return nil
+}
+
+func (r *NodeRegistry) Forget(ctx context.Context, id string) error {
+	if err := r.repo.Delete(ctx, id); err != nil {
+		return fmt.Errorf("forget node: %w", err)
+	}
+	r.log.Info("node removed from registry", slog.String("node_id", id))
+	return nil
+}
+
+func (r *NodeRegistry) List(ctx context.Context) ([]*model.Node, error) {
+	return r.repo.List(ctx)
+}
+
+func (r *NodeRegistry) Get(ctx context.Context, id string) (*model.Node, error) {
+	return r.repo.Get(ctx, id)
+}