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/cell.go

@@ -0,0 +1,109 @@
+package kg
+
+import (
+	"context"
+	"database/sql"
+	"fmt"
+	"time"
+
+	"git.wntrmute.dev/kyle/exo/core"
+	"git.wntrmute.dev/kyle/exo/db"
+)
+
+// CellType distinguishes content types within a note.
+type CellType string
+
+const (
+	CellTypeMarkdown CellType = "markdown"
+	CellTypeCode     CellType = "code"
+	CellTypePlain    CellType = "plain"
+)
+
+// Cell is a content unit within a note. A note is composed of
+// multiple cells of different types.
+type Cell struct {
+	ID       string
+	NodeID   string
+	Type     CellType
+	Contents []byte
+	Ordinal  int
+	Created  time.Time
+	Modified time.Time
+}
+
+// NewCell creates a Cell with a new UUID and current timestamps.
+func NewCell(nodeID string, cellType CellType, contents []byte) *Cell {
+	now := time.Now().UTC()
+	return &Cell{
+		ID:       core.NewUUID(),
+		NodeID:   nodeID,
+		Type:     cellType,
+		Contents: contents,
+		Created:  now,
+		Modified: now,
+	}
+}
+
+// Store persists a Cell to the database.
+func (c *Cell) Store(ctx context.Context, tx *sql.Tx) error {
+	_, err := tx.ExecContext(ctx,
+		`INSERT INTO cells (id, node_id, type, contents, ordinal, created, modified) VALUES (?, ?, ?, ?, ?, ?, ?)`,
+		c.ID, c.NodeID, string(c.Type), c.Contents, c.Ordinal, db.ToDBTime(c.Created), db.ToDBTime(c.Modified))
+	if err != nil {
+		return fmt.Errorf("kg: failed to store cell: %w", err)
+	}
+	return nil
+}
+
+// Get retrieves a Cell by its ID.
+func (c *Cell) Get(ctx context.Context, tx *sql.Tx) error {
+	if c.ID == "" {
+		return fmt.Errorf("kg: cell missing ID: %w", core.ErrNoID)
+	}
+
+	var cellType, created, modified string
+	row := tx.QueryRowContext(ctx,
+		`SELECT node_id, type, contents, ordinal, created, modified FROM cells WHERE id=?`, c.ID)
+	if err := row.Scan(&c.NodeID, &cellType, &c.Contents, &c.Ordinal, &created, &modified); err != nil {
+		return fmt.Errorf("kg: failed to retrieve cell: %w", err)
+	}
+	c.Type = CellType(cellType)
+
+	var err error
+	c.Created, err = db.FromDBTime(created, nil)
+	if err != nil {
+		return err
+	}
+	c.Modified, err = db.FromDBTime(modified, nil)
+	return err
+}
+
+// GetCellsForNode retrieves all cells for a node, ordered by ordinal.
+func GetCellsForNode(ctx context.Context, tx *sql.Tx, nodeID string) ([]*Cell, error) {
+	rows, err := tx.QueryContext(ctx,
+		`SELECT id, type, contents, ordinal, created, modified FROM cells WHERE node_id=? ORDER BY ordinal`, nodeID)
+	if err != nil {
+		return nil, fmt.Errorf("kg: failed to retrieve cells for node: %w", err)
+	}
+	defer func() { _ = rows.Close() }()
+
+	var cells []*Cell
+	for rows.Next() {
+		c := &Cell{NodeID: nodeID}
+		var cellType, created, modified string
+		if err := rows.Scan(&c.ID, &cellType, &c.Contents, &c.Ordinal, &created, &modified); err != nil {
+			return nil, fmt.Errorf("kg: failed to scan cell: %w", err)
+		}
+		c.Type = CellType(cellType)
+		c.Created, err = db.FromDBTime(created, nil)
+		if err != nil {
+			return nil, err
+		}
+		c.Modified, err = db.FromDBTime(modified, nil)
+		if err != nil {
+			return nil, err
+		}
+		cells = append(cells, c)
+	}
+	return cells, rows.Err()
+}