feat(deps): Auto-install beads (bd) when missing (GHI #22)

- Add internal/deps package for dependency management
- Check for bd before gt install and gt rig add
- Auto-install bd via go install if missing
- Version check warns if bd is too old (min: 0.43.0)

Closes #22

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

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

internal/deps/beads_test.go

@@ -0,0 +1,61 @@
+package deps
+
+import "testing"
+
+func TestParseBeadsVersion(t *testing.T) {
+	tests := []struct {
+		input    string
+		expected string
+	}{
+		{"bd version 0.43.0 (dev: main@3e1378e122c6)", "0.43.0"},
+		{"bd version 0.43.0", "0.43.0"},
+		{"bd version 1.2.3", "1.2.3"},
+		{"bd version 10.20.30 (release)", "10.20.30"},
+		{"some other output", ""},
+		{"", ""},
+	}
+
+	for _, tt := range tests {
+		result := parseBeadsVersion(tt.input)
+		if result != tt.expected {
+			t.Errorf("parseBeadsVersion(%q) = %q, want %q", tt.input, result, tt.expected)
+		}
+	}
+}
+
+func TestCompareVersions(t *testing.T) {
+	tests := []struct {
+		a, b     string
+		expected int
+	}{
+		{"0.43.0", "0.43.0", 0},
+		{"0.43.0", "0.42.0", 1},
+		{"0.42.0", "0.43.0", -1},
+		{"1.0.0", "0.99.99", 1},
+		{"0.43.1", "0.43.0", 1},
+		{"0.43.0", "0.43.1", -1},
+	}
+
+	for _, tt := range tests {
+		result := compareVersions(tt.a, tt.b)
+		if result != tt.expected {
+			t.Errorf("compareVersions(%q, %q) = %d, want %d", tt.a, tt.b, result, tt.expected)
+		}
+	}
+}
+
+func TestCheckBeads(t *testing.T) {
+	// This test depends on whether bd is installed in the test environment
+	status, version := CheckBeads()
+
+	// We expect bd to be installed in dev environment
+	if status == BeadsNotFound {
+		t.Skip("bd not installed, skipping integration test")
+	}
+
+	if status == BeadsOK && version == "" {
+		t.Error("CheckBeads returned BeadsOK but empty version")
+	}
+
+	t.Logf("CheckBeads: status=%d, version=%s", status, version)
+}