Remove silent prefix detection from args path in init

When explicit branch names containing slashes were passed to `gh stack
init` (e.g. `gh stack init myprefix/branch`), detectPrefix would
silently extract the prefix and store it in the stack config. This
caused `gh stack add otherbranch` to unexpectedly produce
`myprefix/otherbranch` without the user ever opting in.

Remove the automatic prefix detection from the args path so that
explicit branch names are taken literally. Users who want a prefix
should use `--prefix`. The interactive path (no args) continues to
prompt for confirmation before setting a prefix.

Author: skarim

cmd/init.go

@@ -149,13 +149,6 @@ func runInit(cfg *config.Config, opts *initOptions) error {
 			return err
 		}
 
-		// Prefix detection (only when --prefix not explicitly set)
-		if opts.prefix == "" {
-			if detected := detectPrefix(branches); detected != "" {
-				opts.prefix = detected
-			}
-		}
-
 	} else if opts.numbered {
 		// === NUMBERED PATH (unchanged) ===
 		if opts.prefix == "" && cfg.IsInteractive() {

cmd/init_test.go

@@ -465,7 +465,9 @@ func TestInit_ImplicitAdopt_Mixed(t *testing.T) {
 }
 
 func TestInit_PrefixDetection_ArgsCommonPrefix(t *testing.T) {
-	// Scenario 9: args all share prefix → set silently
+	// Explicit branch names with a common prefix should NOT auto-detect
+	// a prefix — the slash is part of the branch name, not a convention.
+	// Users who want a prefix should use --prefix.
 	gitDir := t.TempDir()
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
@@ -481,7 +483,7 @@ func TestInit_PrefixDetection_ArgsCommonPrefix(t *testing.T) {
 
 	require.NoError(t, err)
 	sf, _ := stack.Load(gitDir)
-	assert.Equal(t, "feat", sf.Stacks[0].Prefix)
+	assert.Equal(t, "", sf.Stacks[0].Prefix)
 }
 
 func TestInit_PrefixDetection_ArgsMixedPrefix(t *testing.T) {
@@ -525,7 +527,8 @@ func TestInit_PrefixDetection_ArgsNoSlash(t *testing.T) {
 }
 
 func TestInit_PrefixDetection_NestedPrefix(t *testing.T) {
-	// Scenario 6: sameen/feat/x → prefix "sameen/feat"
+	// Explicit branch names with nested slashes should NOT auto-detect
+	// a prefix — the user typed the full branch name deliberately.
 	gitDir := t.TempDir()
 	restore := git.SetOps(&git.MockOps{
 		GitDirFn:        func() (string, error) { return gitDir, nil },
@@ -541,7 +544,7 @@ func TestInit_PrefixDetection_NestedPrefix(t *testing.T) {
 
 	require.NoError(t, err)
 	sf, _ := stack.Load(gitDir)
-	assert.Equal(t, "sameen/feat", sf.Stacks[0].Prefix)
+	assert.Equal(t, "", sf.Stacks[0].Prefix)
 }
 
 func TestInit_ExplicitPrefixSkipsDetection(t *testing.T) {