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/placements.go

@@ -0,0 +1,99 @@
+package masterdb
+
+import (
+	"database/sql"
+	"fmt"
+	"time"
+)
+
+// Placement records which node hosts which service.
+type Placement struct {
+	ServiceName string
+	Node        string
+	Tier        string
+	DeployedAt  time.Time
+}
+
+// CreatePlacement inserts or replaces a placement record.
+func CreatePlacement(db *sql.DB, serviceName, node, tier string) error {
+	_, err := db.Exec(`
+		INSERT INTO placements (service_name, node, tier, deployed_at)
+		VALUES (?, ?, ?, datetime('now'))
+		ON CONFLICT(service_name) DO UPDATE SET
+			node = excluded.node,
+			tier = excluded.tier,
+			deployed_at = datetime('now')
+	`, serviceName, node, tier)
+	if err != nil {
+		return fmt.Errorf("create placement %s: %w", serviceName, err)
+	}
+	return nil
+}
+
+// GetPlacement returns the placement for a service.
+func GetPlacement(db *sql.DB, serviceName string) (*Placement, error) {
+	var p Placement
+	var deployedAt string
+	err := db.QueryRow(`
+		SELECT service_name, node, tier, deployed_at
+		FROM placements WHERE service_name = ?
+	`, serviceName).Scan(&p.ServiceName, &p.Node, &p.Tier, &deployedAt)
+	if err == sql.ErrNoRows {
+		return nil, nil
+	}
+	if err != nil {
+		return nil, fmt.Errorf("get placement %s: %w", serviceName, err)
+	}
+	p.DeployedAt, _ = time.Parse("2006-01-02 15:04:05", deployedAt)
+	return &p, nil
+}
+
+// ListPlacements returns all placements.
+func ListPlacements(db *sql.DB) ([]*Placement, error) {
+	rows, err := db.Query(`SELECT service_name, node, tier, deployed_at FROM placements ORDER BY service_name`)
+	if err != nil {
+		return nil, fmt.Errorf("list placements: %w", err)
+	}
+	defer func() { _ = rows.Close() }()
+
+	var placements []*Placement
+	for rows.Next() {
+		var p Placement
+		var deployedAt string
+		if err := rows.Scan(&p.ServiceName, &p.Node, &p.Tier, &deployedAt); err != nil {
+			return nil, fmt.Errorf("scan placement: %w", err)
+		}
+		p.DeployedAt, _ = time.Parse("2006-01-02 15:04:05", deployedAt)
+		placements = append(placements, &p)
+	}
+	return placements, rows.Err()
+}
+
+// DeletePlacement removes a placement record.
+func DeletePlacement(db *sql.DB, serviceName string) error {
+	_, err := db.Exec(`DELETE FROM placements WHERE service_name = ?`, serviceName)
+	if err != nil {
+		return fmt.Errorf("delete placement %s: %w", serviceName, err)
+	}
+	return nil
+}
+
+// CountPlacementsPerNode returns a map of node name → number of placed services.
+func CountPlacementsPerNode(db *sql.DB) (map[string]int, error) {
+	rows, err := db.Query(`SELECT node, COUNT(*) FROM placements GROUP BY node`)
+	if err != nil {
+		return nil, fmt.Errorf("count placements: %w", err)
+	}
+	defer func() { _ = rows.Close() }()
+
+	counts := make(map[string]int)
+	for rows.Next() {
+		var node string
+		var count int
+		if err := rows.Scan(&node, &count); err != nil {
+			return nil, fmt.Errorf("scan count: %w", err)
+		}
+		counts[node] = count
+	}
+	return counts, rows.Err()
+}