test: add docstrings to rewritten tests and use t.Setenv

Addresses two CodeRabbit feedback items on PR #24:

* Docstring coverage warning (was 57.14%, threshold 80%): adds godoc
  comments to the four test functions added or substantially rewritten
  in this PR — TestLoadAndSaveManifest, TestAddAndRemoveFromManifest,
  TestFindInManifest, TestHighlightFileTokensInjectsANSI.
* Quick-win nitpick: replaces the manual os.Setenv/os.Unsetenv +
  defer pattern in TestFindInManifest with t.Setenv, which restores
  the env var automatically on cleanup even on panic or t.Fatal.

go test -race ./... still passes.

internal/extensions/installer_test.go

@@ -245,6 +245,9 @@ func TestManifestEntryIdentity(t *testing.T) {
 	}
 }
 
+// TestLoadAndSaveManifest exercises the live *Installer.loadManifest /
+// saveManifest round-trip against a temp directory, ensuring an absent
+// manifest loads as empty and a saved manifest reads back identically.
 func TestLoadAndSaveManifest(t *testing.T) {
 	tempDir := t.TempDir()
 	installer := &Installer{
@@ -300,6 +303,9 @@ func TestLoadAndSaveManifest(t *testing.T) {
 	}
 }
 
+// TestAddAndRemoveFromManifest verifies that *Installer.addToManifest
+// followed by removeFromManifest leaves the manifest in its original
+// (empty) state, using a temp-directory installer scope.
 func TestAddAndRemoveFromManifest(t *testing.T) {
 	tempDir := t.TempDir()
 	installer := &Installer{
@@ -343,16 +349,13 @@ func TestAddAndRemoveFromManifest(t *testing.T) {
 	}
 }
 
+// TestFindInManifest writes a manifest file directly to the path
+// resolved by the package-level manifestPathForScope helper and then
+// confirms FindInManifest locates the entry by identity (and returns
+// nil for a non-existent identity).
 func TestFindInManifest(t *testing.T) {
 	tempDir := t.TempDir()
-	if err := os.Setenv("XDG_DATA_HOME", tempDir); err != nil {
-		t.Fatalf("Setenv() error = %v", err)
-	}
-	defer func() {
-		if err := os.Unsetenv("XDG_DATA_HOME"); err != nil {
-			t.Logf("Unsetenv() error = %v", err)
-		}
-	}()
+	t.Setenv("XDG_DATA_HOME", tempDir)
 
 	// Write a manifest entry directly via the package-level path resolver
 	// so FindInManifest (which uses manifestPathForScope) can read it back.

internal/ui/render/blocks_test.go

@@ -67,6 +67,9 @@ func TestHighlightFileTokens(t *testing.T) {
 	}
 }
 
+// TestHighlightFileTokensInjectsANSI verifies that HighlightFileTokens
+// preserves the original @file references in the output and wraps each
+// token with ANSI escape codes for the theme accent color.
 func TestHighlightFileTokensInjectsANSI(t *testing.T) {
 	theme := style.DefaultTheme()