Add 'last touched' issue tracking for update/close without ID

When no issue ID is provided to `bd update` or `bd close`, use the last
touched issue from the most recent create, update, show, or close operation.

This addresses the common workflow where you create an issue and then
immediately want to add more details (like changing priority from P2 to P4)
without re-typing the issue ID.

Implementation:
- New file last_touched.go with Get/Set/Clear functions
- Store last touched ID in .beads/last-touched (gitignored)
- Track on create, update, show, and close operations
- Allow update/close with zero args to use last touched

(bd-s2t)

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

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

cmd/bd/last_touched_test.go

@@ -0,0 +1,105 @@
+package main
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestLastTouchedBasic(t *testing.T) {
+	// Create a temp directory to simulate .beads
+	tmpDir := t.TempDir()
+	beadsDir := filepath.Join(tmpDir, ".beads")
+	if err := os.MkdirAll(beadsDir, 0755); err != nil {
+		t.Fatal(err)
+	}
+
+	// Create a marker file so FindBeadsDir recognizes this as a valid beads directory
+	if err := os.WriteFile(filepath.Join(beadsDir, "config.yaml"), []byte{}, 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	// Save the original working directory
+	origDir, err := os.Getwd()
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer func() {
+		_ = os.Chdir(origDir)
+	}()
+
+	// Change to temp directory so FindBeadsDir finds our .beads
+	if err := os.Chdir(tmpDir); err != nil {
+		t.Fatal(err)
+	}
+
+	// Test that no last touched returns empty
+	got := GetLastTouchedID()
+	if got != "" {
+		t.Errorf("GetLastTouchedID() = %q, want empty", got)
+	}
+
+	// Set and retrieve
+	testID := "bd-test123"
+	SetLastTouchedID(testID)
+	got = GetLastTouchedID()
+	if got != testID {
+		t.Errorf("GetLastTouchedID() = %q, want %q", got, testID)
+	}
+
+	// Update with new ID
+	testID2 := "bd-test456"
+	SetLastTouchedID(testID2)
+	got = GetLastTouchedID()
+	if got != testID2 {
+		t.Errorf("GetLastTouchedID() = %q, want %q", got, testID2)
+	}
+
+	// Clear and verify
+	ClearLastTouched()
+	got = GetLastTouchedID()
+	if got != "" {
+		t.Errorf("After ClearLastTouched(), GetLastTouchedID() = %q, want empty", got)
+	}
+}
+
+func TestSetLastTouchedIDIgnoresEmpty(t *testing.T) {
+	// Create a temp directory
+	tmpDir := t.TempDir()
+	beadsDir := filepath.Join(tmpDir, ".beads")
+	if err := os.MkdirAll(beadsDir, 0755); err != nil {
+		t.Fatal(err)
+	}
+
+	// Create a marker file so FindBeadsDir recognizes this as a valid beads directory
+	if err := os.WriteFile(filepath.Join(beadsDir, "config.yaml"), []byte{}, 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	// Save the original working directory
+	origDir, err := os.Getwd()
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer func() {
+		_ = os.Chdir(origDir)
+	}()
+
+	// Change to temp directory
+	if err := os.Chdir(tmpDir); err != nil {
+		t.Fatal(err)
+	}
+
+	// First set a value
+	testID := "bd-original"
+	SetLastTouchedID(testID)
+
+	// Try to set empty - should be ignored
+	SetLastTouchedID("")
+
+	// Should still have original value
+	got := GetLastTouchedID()
+	if got != testID {
+		t.Errorf("After SetLastTouchedID(\"\"), GetLastTouchedID() = %q, want %q", got, testID)
+	}
+}