Refuse fetch writes through a symlink in the destination (closes #86)

sanitizePath checks manifest paths only as text, so a symlink already
inside the destination directory could send fetch's writes outside it.
checkNoSymlinks now looks at each existing part of a path with os.Lstat
and refuses the path if any part is a symlink, wherever it points.
fetch runs it immediately before each write: creating the parent
directories, creating the temp file, and renaming it into place. The new
test puts such a symlink at each of those three places, and once inside
a plain directory, and checks that the fetch fails and nothing outside
changes. The G304 comment now states what holds. A symlink swapped in
between a check and its write is not caught; os.Root closes that once
the Go version is raised.

Model: opus-5-5

internal/cli/fetch.go

@@ -295,7 +295,7 @@ func checkNoSymlinks(p string) error {
 		}
 
 		if err != nil {
-			return err
+			return fmt.Errorf("failed to check %s for a symlink: %w", current, err)
 		}
 
 		if info.Mode()&os.ModeSymlink != 0 {

internal/cli/fetch_test.go

@@ -444,8 +444,8 @@ func TestFetchProgress(t *testing.T) {
 // TestFetchRefusesSymlinks runs fetch into a destination directory that
 // holds a symlink pointing outside it, in each of the three places fetch
 // writes: a parent directory, the temp file, and the file itself, which
-// the temp file is renamed onto. The fetch must fail and nothing outside
-// may change.
+// the temp file is renamed onto; and once as a directory inside a plain
+// directory. The fetch must fail and nothing outside may change.
 //
 //nolint:paralleltest // changes the process-global working directory
 func TestFetchRefusesSymlinks(t *testing.T) {
@@ -456,6 +456,7 @@ func TestFetchRefusesSymlinks(t *testing.T) {
 		target string // what link points to, relative to the outside directory
 	}{
 		{"parent directory", "sub/deeper/file.txt", "sub", "."},
+		{"directory inside a plain directory", "docs/data/passwd", "docs/data", "."},
 		{"temp file", testFileTxt, ".file.txt.tmp", "new.txt"},
 		{"file", testFileTxt, testFileTxt, "new.txt"},
 	}
@@ -474,6 +475,7 @@ func TestFetchRefusesSymlinks(t *testing.T) {
 			outside := t.TempDir()
 
 			chdirTemp(t)
+			require.NoError(t, os.MkdirAll(filepath.Dir(tt.link), 0o755))
 			require.NoError(t, os.Symlink(filepath.Join(outside, tt.target), tt.link))
 
 			opts := testOpts([]string{testApp, "fetch", "-q", server.URL}, afero.NewOsFs())