feat: add bd slot commands for agent bead slot management (gt-h5sza)

Add slot management commands:
- bd slot set <agent> <slot> <bead> - set slot (error if occupied)
- bd slot clear <agent> <slot> - clear slot
- bd slot show <agent> - show all slots

These enforce cardinality constraints for agent bead slots.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/storage/sqlite/migrations/030_agent_fields.go

@@ -0,0 +1,53 @@
+package migrations
+
+import (
+	"database/sql"
+	"fmt"
+)
+
+// MigrateAgentFields adds agent-specific fields to the issues table.
+// These fields support the agent-as-bead pattern (gt-v2gkv, gt-h5sza):
+//   - hook_bead: current work attached to agent's hook (0..1 cardinality)
+//   - role_bead: reference to role definition bead
+//   - agent_state: agent-reported state (idle|running|stuck|stopped)
+//   - last_activity: timestamp for timeout detection
+//   - role_type: agent role (polecat|crew|witness|refinery|mayor|deacon)
+//   - rig: rig name (empty for town-level agents)
+func MigrateAgentFields(db *sql.DB) error {
+	columns := []struct {
+		name    string
+		sqlType string
+	}{
+		{"hook_bead", "TEXT DEFAULT ''"},
+		{"role_bead", "TEXT DEFAULT ''"},
+		{"agent_state", "TEXT DEFAULT ''"},
+		{"last_activity", "DATETIME"},
+		{"role_type", "TEXT DEFAULT ''"},
+		{"rig", "TEXT DEFAULT ''"},
+	}
+
+	for _, col := range columns {
+		// Check if column already exists
+		var columnExists bool
+		err := db.QueryRow(`
+			SELECT COUNT(*) > 0
+			FROM pragma_table_info('issues')
+			WHERE name = ?
+		`, col.name).Scan(&columnExists)
+		if err != nil {
+			return fmt.Errorf("failed to check %s column: %w", col.name, err)
+		}
+
+		if columnExists {
+			continue
+		}
+
+		// Add the column
+		_, err = db.Exec(fmt.Sprintf(`ALTER TABLE issues ADD COLUMN %s %s`, col.name, col.sqlType))
+		if err != nil {
+			return fmt.Errorf("failed to add %s column: %w", col.name, err)
+		}
+	}
+
+	return nil
+}