Add Phase 4 knowledge graph: nodes, cells, facts, edges, gRPC service

Build the knowledge graph pillar with the kg package:
- Node: hierarchical notes with parent/children, C2 wiki-style naming,
  shared tag/category pool with artifacts
- Cell: content units (markdown, code, plain) with ordinal ordering
- Fact: EAV tuples with transaction timestamps and retraction support
- Edge: directed graph links (child, parent, related, artifact_link)

Includes schema migration (002_knowledge_graph.sql), protobuf definitions
(kg.proto), full gRPC KnowledgeGraphService implementation, CLI commands
(node create/get), and comprehensive test coverage.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

kg/edge.go

@@ -0,0 +1,104 @@
+package kg
+
+import (
+	"context"
+	"database/sql"
+	"fmt"
+	"time"
+
+	"git.wntrmute.dev/kyle/exo/core"
+	"git.wntrmute.dev/kyle/exo/db"
+)
+
+// EdgeRelation describes the type of relationship between two nodes.
+type EdgeRelation string
+
+const (
+	EdgeRelationChild        EdgeRelation = "child"
+	EdgeRelationParent       EdgeRelation = "parent"
+	EdgeRelationRelated      EdgeRelation = "related"
+	EdgeRelationArtifactLink EdgeRelation = "artifact_link"
+)
+
+// Edge links a source node to a target node or artifact.
+type Edge struct {
+	ID       string
+	SourceID string
+	TargetID string
+	Relation EdgeRelation
+	Created  time.Time
+}
+
+// NewEdge creates an Edge with a new UUID and current timestamp.
+func NewEdge(sourceID, targetID string, relation EdgeRelation) *Edge {
+	return &Edge{
+		ID:       core.NewUUID(),
+		SourceID: sourceID,
+		TargetID: targetID,
+		Relation: relation,
+		Created:  time.Now().UTC(),
+	}
+}
+
+// Store persists an Edge to the database.
+func (e *Edge) Store(ctx context.Context, tx *sql.Tx) error {
+	_, err := tx.ExecContext(ctx,
+		`INSERT INTO edges (id, source_id, target_id, relation, created) VALUES (?, ?, ?, ?, ?)`,
+		e.ID, e.SourceID, e.TargetID, string(e.Relation), db.ToDBTime(e.Created))
+	if err != nil {
+		return fmt.Errorf("kg: failed to store edge: %w", err)
+	}
+	return nil
+}
+
+// GetEdgesFrom retrieves all edges originating from a given node.
+func GetEdgesFrom(ctx context.Context, tx *sql.Tx, sourceID string) ([]*Edge, error) {
+	rows, err := tx.QueryContext(ctx,
+		`SELECT id, target_id, relation, created FROM edges WHERE source_id=?`, sourceID)
+	if err != nil {
+		return nil, fmt.Errorf("kg: failed to retrieve edges: %w", err)
+	}
+	defer func() { _ = rows.Close() }()
+
+	var edges []*Edge
+	for rows.Next() {
+		e := &Edge{SourceID: sourceID}
+		var relation, created string
+		if err := rows.Scan(&e.ID, &e.TargetID, &relation, &created); err != nil {
+			return nil, fmt.Errorf("kg: failed to scan edge: %w", err)
+		}
+		e.Relation = EdgeRelation(relation)
+		e.Created, err = db.FromDBTime(created, nil)
+		if err != nil {
+			return nil, err
+		}
+		edges = append(edges, e)
+	}
+	return edges, rows.Err()
+}
+
+// GetEdgesTo retrieves all edges pointing to a given node/artifact.
+func GetEdgesTo(ctx context.Context, tx *sql.Tx, targetID string) ([]*Edge, error) {
+	rows, err := tx.QueryContext(ctx,
+		`SELECT id, source_id, relation, created FROM edges WHERE target_id=?`, targetID)
+	if err != nil {
+		return nil, fmt.Errorf("kg: failed to retrieve incoming edges: %w", err)
+	}
+	defer func() { _ = rows.Close() }()
+
+	var edges []*Edge
+	for rows.Next() {
+		e := &Edge{TargetID: targetID}
+		var relation, created string
+		if err := rows.Scan(&e.ID, &e.SourceID, &relation, &created); err != nil {
+			return nil, fmt.Errorf("kg: failed to scan edge: %w", err)
+		}
+		e.Relation = EdgeRelation(relation)
+		e.Created, err = db.FromDBTime(created, nil)
+		if err != nil {
+			return nil, err
+		}
+		edges = append(edges, e)
+	}
+	return edges, rows.Err()
+}