Add master database with nodes, placements, and edge_routes

New internal/masterdb/ package for mcp-master cluster state. Separate
from the agent's registry because the schemas are fundamentally
different (cluster-wide placement vs node-local containers).

Tables: nodes, placements, edge_routes. Full CRUD with tests.
Follows the same Open/migrate pattern as internal/registry/.

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

internal/masterdb/nodes.go

@@ -0,0 +1,103 @@
+package masterdb
+
+import (
+	"database/sql"
+	"fmt"
+	"time"
+)
+
+// Node represents a registered node in the cluster.
+type Node struct {
+	Name          string
+	Address       string
+	Role          string
+	Arch          string
+	Status        string
+	Containers    int
+	LastHeartbeat *time.Time
+}
+
+// UpsertNode inserts or updates a node in the registry.
+func UpsertNode(db *sql.DB, name, address, role, arch string) error {
+	_, err := db.Exec(`
+		INSERT INTO nodes (name, address, role, arch, updated_at)
+		VALUES (?, ?, ?, ?, datetime('now'))
+		ON CONFLICT(name) DO UPDATE SET
+			address = excluded.address,
+			role = excluded.role,
+			arch = excluded.arch,
+			updated_at = datetime('now')
+	`, name, address, role, arch)
+	if err != nil {
+		return fmt.Errorf("upsert node %s: %w", name, err)
+	}
+	return nil
+}
+
+// GetNode returns a single node by name.
+func GetNode(db *sql.DB, name string) (*Node, error) {
+	var n Node
+	var lastHB sql.NullString
+	err := db.QueryRow(`
+		SELECT name, address, role, arch, status, containers, last_heartbeat
+		FROM nodes WHERE name = ?
+	`, name).Scan(&n.Name, &n.Address, &n.Role, &n.Arch, &n.Status, &n.Containers, &lastHB)
+	if err == sql.ErrNoRows {
+		return nil, nil
+	}
+	if err != nil {
+		return nil, fmt.Errorf("get node %s: %w", name, err)
+	}
+	if lastHB.Valid {
+		t, _ := time.Parse("2006-01-02 15:04:05", lastHB.String)
+		n.LastHeartbeat = &t
+	}
+	return &n, nil
+}
+
+// ListNodes returns all nodes.
+func ListNodes(db *sql.DB) ([]*Node, error) {
+	return queryNodes(db, `SELECT name, address, role, arch, status, containers, last_heartbeat FROM nodes ORDER BY name`)
+}
+
+// ListWorkerNodes returns nodes with role "worker" or "master" (master is also a worker).
+func ListWorkerNodes(db *sql.DB) ([]*Node, error) {
+	return queryNodes(db, `SELECT name, address, role, arch, status, containers, last_heartbeat FROM nodes WHERE role IN ('worker', 'master') ORDER BY name`)
+}
+
+// ListEdgeNodes returns nodes with role "edge".
+func ListEdgeNodes(db *sql.DB) ([]*Node, error) {
+	return queryNodes(db, `SELECT name, address, role, arch, status, containers, last_heartbeat FROM nodes WHERE role = 'edge' ORDER BY name`)
+}
+
+func queryNodes(db *sql.DB, query string) ([]*Node, error) {
+	rows, err := db.Query(query)
+	if err != nil {
+		return nil, fmt.Errorf("query nodes: %w", err)
+	}
+	defer func() { _ = rows.Close() }()
+
+	var nodes []*Node
+	for rows.Next() {
+		var n Node
+		var lastHB sql.NullString
+		if err := rows.Scan(&n.Name, &n.Address, &n.Role, &n.Arch, &n.Status, &n.Containers, &lastHB); err != nil {
+			return nil, fmt.Errorf("scan node: %w", err)
+		}
+		if lastHB.Valid {
+			t, _ := time.Parse("2006-01-02 15:04:05", lastHB.String)
+			n.LastHeartbeat = &t
+		}
+		nodes = append(nodes, &n)
+	}
+	return nodes, rows.Err()
+}
+
+// UpdateNodeStatus updates a node's status field.
+func UpdateNodeStatus(db *sql.DB, name, status string) error {
+	_, err := db.Exec(`UPDATE nodes SET status = ?, updated_at = datetime('now') WHERE name = ?`, status, name)
+	if err != nil {
+		return fmt.Errorf("update node status %s: %w", name, err)
+	}
+	return nil
+}