feat(daemon): add BD_SOCKET env var for test isolation (#914)

Add BD_SOCKET environment variable support to override daemon socket path,
enabling parallel test isolation via t.TempDir() + t.Setenv().

Changes:
- getSocketPath() checks BD_SOCKET first, falls back to dbPath-derived path
- getSocketPathForPID() checks BD_SOCKET first (for consistency)
- Add daemon_socket_test.go with isolation pattern examples

This is a minimal tracer bullet to validate the approach before
expanding to full test isolation infrastructure.

Backward compatible: default behavior unchanged without env var set.

cmd/bd/daemon_socket_test.go

@@ -0,0 +1,75 @@
+package main
+
+import (
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+// TestSocketPathEnvOverride verifies that BD_SOCKET env var overrides default socket path.
+func TestSocketPathEnvOverride(t *testing.T) {
+	// Create isolated temp directory
+	tmpDir := t.TempDir()
+	customSocket := filepath.Join(tmpDir, "custom.sock")
+
+	// Set environment for isolation
+	t.Setenv("BD_SOCKET", customSocket)
+
+	// Verify getSocketPath returns custom path
+	got := getSocketPath()
+	assert.Equal(t, customSocket, got)
+}
+
+// TestSocketPathForPIDEnvOverride verifies that BD_SOCKET env var overrides PID-derived path.
+func TestSocketPathForPIDEnvOverride(t *testing.T) {
+	// Create isolated temp directory
+	tmpDir := t.TempDir()
+	customSocket := filepath.Join(tmpDir, "custom.sock")
+
+	// Set environment for isolation
+	t.Setenv("BD_SOCKET", customSocket)
+
+	// Verify getSocketPathForPID returns custom path (ignoring pidFile)
+	pidFile := "/some/other/path/daemon.pid"
+	got := getSocketPathForPID(pidFile)
+	assert.Equal(t, customSocket, got)
+}
+
+// TestSocketPathDefaultBehavior verifies default behavior when BD_SOCKET is not set.
+func TestSocketPathDefaultBehavior(t *testing.T) {
+	// Ensure BD_SOCKET is not set (t.Setenv restores after test)
+	t.Setenv("BD_SOCKET", "")
+
+	// Verify getSocketPathForPID derives from PID file path
+	pidFile := "/path/to/.beads/daemon.pid"
+	got := getSocketPathForPID(pidFile)
+	assert.Equal(t, "/path/to/.beads/bd.sock", got)
+}
+
+// TestDaemonSocketIsolation demonstrates that two test instances can use different sockets.
+// This is the key pattern for parallel test isolation.
+func TestDaemonSocketIsolation(t *testing.T) {
+	// Simulate two parallel tests with different socket paths
+	tests := []struct {
+		name       string
+		sockSuffix string
+	}{
+		{"instance_a", "a.sock"},
+		{"instance_b", "b.sock"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			// Each sub-test gets isolated socket path in its own temp dir
+			socketPath := filepath.Join(t.TempDir(), tt.sockSuffix)
+			t.Setenv("BD_SOCKET", socketPath)
+
+			got := getSocketPath()
+			assert.Equal(t, socketPath, got)
+
+			// Verify paths are unique per instance
+			assert.Contains(t, got, tt.sockSuffix)
+		})
+	}
+}