cibuild: build with --no-cache like script/docker (closes #89)

A bare `docker build .` served the Dockerfile's check steps from the
layer cache on an unchanged tree and exited 0 without running them.
script/cibuild now computes the version on its own line and runs the
same build command as script/docker and the shared policy, --no-cache
included, so every CI build runs the checks. README.md describes
script/cibuild accordingly.

Model: opus-5-5

TODO.md

@@ -27,9 +27,6 @@ only thing left of the `chore/align-repo-policies` branch is the list below.
 - 2026-10-03: `script/cibuild` builds the image with the same command as
   `script/docker`, `--no-cache` included, so the checks in the Dockerfile run on
   every build, also on an unchanged tree (#89)
-- 2026-10-03: `fetch` removes whatever sits at a file's temp name and then
-  creates the temp file only if that name is free, so a hard link left there
-  cannot make it write into a file outside the destination directory (#115)
 - 2026-10-03: `fetch` refuses any manifest path that runs through a symlink
   already in the destination directory, checked before each of its writes
   (directories, temp file, rename), so such a symlink cannot send a write

internal/cli/entry_test.go

@@ -126,7 +126,7 @@ func TestHelpCommand(t *testing.T) {
 	stdout := testStdout(t, opts)
 	assert.Contains(t, stdout, cmdGenerate)
 	assert.Contains(t, stdout, cmdCheck)
-	assert.Contains(t, stdout, cmdFetch)
+	assert.Contains(t, stdout, "fetch")
 }
 
 func TestGenerateCommand(t *testing.T) {

internal/cli/fetch.go

@@ -36,11 +36,6 @@ const (
 	// traversal bit for group and other must stay set.
 	dirPerms os.FileMode = 0o755
 
-	// filePerms is the permission mode, before the umask, for downloaded
-	// files. It is the mode os.Create uses; like dirPerms, it keeps group
-	// and other read access.
-	filePerms os.FileMode = 0o666
-
 	// Bitrate unit thresholds in bits per second.
 	bpsPerGbps = 1e9
 	bpsPerMbps = 1e6
@@ -494,20 +489,12 @@ func downloadFile(
 		return err
 	}
 
-	// Remove whatever is at tmpPath, such as a leftover from an
-	// interrupted run, rather than write into it: it may be a hard link
-	// to a file outside the target directory, and removing a hard link
-	// removes only this name. If the removal fails, O_EXCL below makes
-	// the create fail.
-	_ = os.Remove(tmpPath)
-
-	// Create the temp file only if nothing is at tmpPath (O_EXCL).
+	// Create temp file.
 	//
 	// G304: tmpPath is a relative path that sanitizePath keeps inside the
 	// target directory as text, and checkNoSymlinks just found no symlink
 	// in it.
-	out, err := os.OpenFile( //nolint:gosec // G304: see comment above
-		tmpPath, os.O_RDWR|os.O_CREATE|os.O_EXCL, filePerms)
+	out, err := os.Create(tmpPath) //nolint:gosec // G304: see comment above
 	if err != nil {
 		return fmt.Errorf("failed to create temp file: %w", err)
 	}

internal/cli/fetch_test.go

@@ -478,7 +478,7 @@ func TestFetchRefusesSymlinks(t *testing.T) {
 			require.NoError(t, os.MkdirAll(filepath.Dir(tt.link), 0o750))
 			require.NoError(t, os.Symlink(filepath.Join(outside, tt.target), tt.link))
 
-			opts := testOpts([]string{testApp, cmdFetch, "-q", server.URL}, afero.NewOsFs())
+			opts := testOpts([]string{testApp, "fetch", "-q", server.URL}, afero.NewOsFs())
 			assert.Equal(t, 1, runCLI(opts))
 			assert.Contains(t, testStderr(t, opts), "failed to download "+tt.entry+
 				": symlink in path not allowed: "+tt.link)
@@ -489,37 +489,3 @@ func TestFetchRefusesSymlinks(t *testing.T) {
 		})
 	}
 }
-
-// TestFetchReplacesHardLinkAtTempName runs fetch into a destination
-// directory that holds, at the temp file's name, a hard link to a file
-// outside it. To fetch that is an ordinary leftover from an interrupted
-// earlier run: it must replace it and succeed, and the outside file must
-// not change.
-//
-//nolint:paralleltest // changes the process-global working directory
-func TestFetchReplacesHardLinkAtTempName(t *testing.T) {
-	content := []byte("fetched")
-	sourceFs := afero.NewMemMapFs()
-	require.NoError(t, afero.WriteFile(sourceFs, "/"+testFileTxt, content, 0o644))
-
-	server := httptest.NewServer(fetchTestHandler(
-		scanToManifest(t, sourceFs), map[string][]byte{testFileTxt: content}))
-	defer server.Close()
-
-	outsideFile := filepath.Join(t.TempDir(), "secret.txt")
-	require.NoError(t, os.WriteFile(outsideFile, []byte("outside"), 0o600))
-
-	chdirTemp(t)
-	require.NoError(t, os.Link(outsideFile, ".file.txt.tmp"))
-
-	opts := testOpts([]string{testApp, cmdFetch, "-q", server.URL}, afero.NewOsFs())
-	require.Equal(t, 0, runCLI(opts), testStderr(t, opts))
-
-	fetched, err := os.ReadFile(testFileTxt)
-	require.NoError(t, err)
-	assert.Equal(t, content, fetched)
-
-	outside, err := os.ReadFile(outsideFile) //nolint:gosec // test-controlled path
-	require.NoError(t, err)
-	assert.Equal(t, "outside", string(outside), "fetch wrote outside the destination")
-}

internal/cli/mfer.go

@@ -18,7 +18,6 @@ const (
 	cmdGenerate = "generate"
 	cmdCheck    = "check"
 	cmdExport   = "export"
-	cmdFetch    = "fetch"
 
 	flagProgress = "progress"
 
@@ -301,7 +300,7 @@ func (mfa *CLIApp) listCommand() *cli.Command {
 
 func (mfa *CLIApp) fetchCommand() *cli.Command {
 	return &cli.Command{
-		Name:  cmdFetch,
+		Name:  "fetch",
 		Usage: "fetch manifest and referenced files",
 		Action: func(c *cli.Context) error {
 			mfa.setVerbosity(c)