refactor: remove unused bd pin/unpin/hook commands (bd-x0zl)

Analysis found these commands are dead code:
- gt never calls `bd pin` - uses `bd update --status=pinned` instead
- Beads.Pin() wrapper exists but is never called
- bd hook functionality duplicated by gt mol status
- Code comment says "pinned field is cosmetic for bd hook visibility"

Removed:
- cmd/bd/pin.go
- cmd/bd/unpin.go
- cmd/bd/hook.go

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

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

cmd/bd/doctor/config_values_test.go

@@ -213,6 +213,21 @@ func TestCheckMetadataConfigValues(t *testing.T) {
 			t.Error("expected issues for wrong jsonl extension")
 		}
 	})
+
+	t.Run("jsonl_export cannot be system file", func(t *testing.T) {
+		metadataContent := `{
+  "database": "beads.db",
+  "jsonl_export": "interactions.jsonl"
+}`
+		if err := os.WriteFile(filepath.Join(beadsDir, "metadata.json"), []byte(metadataContent), 0644); err != nil {
+			t.Fatalf("failed to write metadata.json: %v", err)
+		}
+
+		issues := checkMetadataConfigValues(tmpDir)
+		if len(issues) == 0 {
+			t.Error("expected issues for system jsonl_export")
+		}
+	})
 }
 
 func contains(s, substr string) bool {