feat(plugin): add plugin discovery, management, and run tracking

- internal/plugin/types.go: Plugin type definitions with TOML frontmatter schema
- internal/plugin/scanner.go: Discover plugins from town and rig directories
- internal/plugin/recording.go: Record plugin runs as ephemeral beads
- internal/cmd/plugin.go: `gt plugin list` and `gt plugin show` commands

Plugin locations: ~/gt/plugins/ (town-level), <rig>/plugins/ (rig-level).
Rig-level plugins override town-level by name.

Closes: gt-h8k4z, gt-rsejc, gt-n08ix.3

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

internal/plugin/recording_test.go

@@ -0,0 +1,50 @@
+package plugin
+
+import (
+	"testing"
+)
+
+func TestPluginRunRecord(t *testing.T) {
+	record := PluginRunRecord{
+		PluginName: "test-plugin",
+		RigName:    "gastown",
+		Result:     ResultSuccess,
+		Body:       "Test run completed successfully",
+	}
+
+	if record.PluginName != "test-plugin" {
+		t.Errorf("expected plugin name 'test-plugin', got %q", record.PluginName)
+	}
+	if record.RigName != "gastown" {
+		t.Errorf("expected rig name 'gastown', got %q", record.RigName)
+	}
+	if record.Result != ResultSuccess {
+		t.Errorf("expected result 'success', got %q", record.Result)
+	}
+}
+
+func TestRunResultConstants(t *testing.T) {
+	if ResultSuccess != "success" {
+		t.Errorf("expected ResultSuccess to be 'success', got %q", ResultSuccess)
+	}
+	if ResultFailure != "failure" {
+		t.Errorf("expected ResultFailure to be 'failure', got %q", ResultFailure)
+	}
+	if ResultSkipped != "skipped" {
+		t.Errorf("expected ResultSkipped to be 'skipped', got %q", ResultSkipped)
+	}
+}
+
+func TestNewRecorder(t *testing.T) {
+	recorder := NewRecorder("/tmp/test-town")
+	if recorder == nil {
+		t.Fatal("NewRecorder returned nil")
+	}
+	if recorder.townRoot != "/tmp/test-town" {
+		t.Errorf("expected townRoot '/tmp/test-town', got %q", recorder.townRoot)
+	}
+}
+
+// Integration tests for RecordRun, GetLastRun, GetRunsSince require
+// a working beads installation and are skipped in unit tests.
+// These functions shell out to `bd` commands.