fix: ホーム/ルートでセッションが無くてもshpoolを起動できるようにする

DEV.md

@@ -101,6 +101,8 @@ internal/tui/select_test.go
 | `/Users/uzulla/プロジェクト/api`  | `______.api-f10f1a6e` (非ASCIIは1文字あたり `_` + suffix) |
 | `/srv/app-v1.2.3/api`              | `srv.app-v1.2.3.api-597dbea5` |
 
+ホームディレクトリそのもの（相対パスが空になる）やファイルシステムのルートでは相対パス由来の名前が空になる。この場合は `session.FallbackName()` が `$HOME` を取り除かないフルパスから名前を生成する（例: `/home/developer` → `home.developer`）。フルパスでも空になるルート `/` のみ `shell` に固定フォールバックする。`shp` (引数なし) / `shp -f` / `shp --print-name` はいずれも `session.FromCwdOrFallback()` を通すので、ホーム/ルートでも空にならず一貫して起動・出力できる。
+
 MVP ではこのルールは固定。将来は `~/work` や `~/src` を root として相対化するなどの設定を追加できる設計にしてある (`session.FromPath(path, home)` で home を引数化済み)。
 
 ## attach の実行方式
@@ -118,7 +120,7 @@ attach には常に `--dir .` を渡す。既存セッションへの attach で
 - 空行を除外
 - `NAME STATUS` のヘッダ行を除外
 - 各行の先頭カラムをセッション名として扱う
-- 候補が0件なら標準エラーに `No shpool sessions found.` を表示して終了
+- 既存セッションが0件でも、cwd 由来名（空ならフルパス由来のフォールバック名）を既定候補として picker を表示し、`shp` がそのまま起動できるようにする
 
 ## TUI
 
@@ -147,8 +149,8 @@ MVP では fuzzy / negative / 正規表現 / 複数選択 / preview pane / sort
 | 状況 | 出力 | 終了コード |
 | --- | --- | --- |
 | `shpool` が PATH に無い | `Error: shpool command not found in PATH. Please install shpool first` | 1 |
-| `shpool list` で一覧取得不可 | cwd 候補だけを表示。cwd 名も空なら `No shpool sessions found.` | 0 |
+| `shpool list` で一覧取得不可 | cwd 候補（空ならフォールバック名）だけを表示 | 0 |
 | `shpool list` 失敗 (一覧取得不可として扱わないもの) | `Error: shpool list failed: <stderr>` | 1 |
-| セッション0件 | `No shpool sessions found.` (stderr) | 0 |
+| セッション0件 | cwd 候補（空ならフォールバック名）だけの picker を表示 | 0 |
 | TUI でキャンセル | (何も出さない) | 0 |
 | 不正な引数 | `Error: too many arguments` 等 + usage | 1 |

cmd/shp/main.go

@@ -30,6 +30,9 @@ Usage:
   shp -h | --help          Show this help.
 
 New sessions are created in the current directory.
+In the home directory (or filesystem root), where no name can be derived from
+a relative path, the session name falls back to one derived from the full path
+(e.g. "home.<user>"), so shp still launches instead of dead-ending.
 shpool must be installed and on PATH.
 `
 
@@ -64,7 +67,7 @@ func run(args []string) error {
 		if len(rest) > 0 {
 			return fmt.Errorf("--print-name takes no arguments")
 		}
-		name, err := session.FromCwd()
+		name, err := session.FromCwdOrFallback()
 		if err != nil {
 			return err
 		}
@@ -81,15 +84,13 @@ func run(args []string) error {
 		return fmt.Errorf("too many arguments")
 	}
 
-	// `shp -f` without a name: force-attach directly to the cwd-derived name.
+	// `shp -f` without a name: force-attach directly to the cwd-derived name,
+	// or the full-path fallback when the cwd yields none (home / root).
 	if force {
-		name, err := session.FromCwd()
+		name, err := session.FromCwdOrFallback()
 		if err != nil {
 			return err
 		}
-		if name == "" {
-			return fmt.Errorf("could not derive a session name from the current directory")
-		}
 		return shpool.Attach(name, true)
 	}
 
@@ -98,7 +99,11 @@ func run(args []string) error {
 }
 
 func runPicker() error {
-	cwdName, err := session.FromCwd()
+	// Use the full-path fallback when the cwd yields no name (the home
+	// directory itself or the filesystem root) so the picker always has a
+	// default create-entry, just like any other directory, instead of
+	// dead-ending with "no sessions".
+	cwdName, err := session.FromCwdOrFallback()
 	if err != nil {
 		return err
 	}
@@ -111,11 +116,6 @@ func runPicker() error {
 		sessions = nil
 	}
 
-	if cwdName == "" && len(sessions) == 0 {
-		fmt.Fprintln(os.Stderr, "No shpool sessions found.")
-		return nil
-	}
-
 	picked, err := tui.SelectWithDefault(sessions, cwdName)
 	if err != nil {
 		if errors.Is(err, tui.ErrCancelled) {

internal/session/name.go

@@ -20,6 +20,40 @@ func FromCwd() (string, error) {
 	return FromPath(cwd, home), nil
 }
 
+// FromCwdOrFallback returns the cwd-derived session name, falling back to
+// FallbackName when the cwd yields no name (the home directory itself or the
+// filesystem root). The result is always non-empty, so callers can attach or
+// print a name without dead-ending.
+func FromCwdOrFallback() (string, error) {
+	name, err := FromCwd()
+	if err != nil {
+		return "", err
+	}
+	if name != "" {
+		return name, nil
+	}
+	return FallbackName()
+}
+
+// FallbackName returns a session name to use when FromCwd yields no name,
+// which happens in the home directory itself or at the filesystem root. It
+// derives the name from the full absolute path (without stripping the home
+// prefix), so the home directory maps to e.g. "home.developer" rather than
+// dead-ending. This keeps the name unique and consistent with FromPath's
+// naming rules. It falls back to "shell" only when even the full path yields
+// nothing (the filesystem root).
+func FallbackName() (string, error) {
+	cwd, err := os.Getwd()
+	if err != nil {
+		return "", fmt.Errorf("get cwd: %w", err)
+	}
+	name := FromPath(cwd, "")
+	if name == "" {
+		name = "shell"
+	}
+	return name, nil
+}
+
 // FromPath converts an absolute path to a session name.
 // If home is non-empty and path is under home, the home prefix is stripped first.
 func FromPath(path, home string) string {

internal/session/name_test.go

@@ -1,6 +1,9 @@
 package session
 
-import "testing"
+import (
+	"os"
+	"testing"
+)
 
 func TestFromPath(t *testing.T) {
 	cases := []struct {
@@ -89,6 +92,81 @@ func TestFromPath_JapaneseLen(t *testing.T) {
 	}
 }
 
+func TestFallbackName_DerivesFromFullPath(t *testing.T) {
+	t.Chdir(t.TempDir())
+
+	got, err := FallbackName()
+	if err != nil {
+		t.Fatalf("FallbackName() error: %v", err)
+	}
+	// The full (home-prefix-unstripped) path is used, so the name must match
+	// FromPath with no home, never be empty, and reflect the directory.
+	// Resolve the working directory via Getwd (TempDir may contain symlinks).
+	dir, err := os.Getwd()
+	if err != nil {
+		t.Fatalf("Getwd error: %v", err)
+	}
+	want := FromPath(dir, "")
+	if got != want {
+		t.Errorf("FallbackName() = %q, want %q", got, want)
+	}
+	if got == "" {
+		t.Errorf("FallbackName() returned empty name for %q", dir)
+	}
+}
+
+func TestFallbackName_RootFallsBackToShell(t *testing.T) {
+	t.Chdir("/")
+
+	got, err := FallbackName()
+	if err != nil {
+		t.Fatalf("FallbackName() error: %v", err)
+	}
+	if got != "shell" {
+		t.Errorf("FallbackName() at root = %q, want %q", got, "shell")
+	}
+}
+
+func TestFromCwdOrFallback_AtHomeUsesFallback(t *testing.T) {
+	home, err := os.UserHomeDir()
+	if err != nil {
+		t.Skipf("no home directory: %v", err)
+	}
+	t.Chdir(home)
+
+	// The real trigger for the fallback: at home, FromCwd derives nothing.
+	if got, _ := FromCwd(); got != "" {
+		t.Fatalf("FromCwd() at home = %q, want empty (precondition for fallback)", got)
+	}
+
+	got, err := FromCwdOrFallback()
+	if err != nil {
+		t.Fatalf("FromCwdOrFallback() error: %v", err)
+	}
+	if got == "" {
+		t.Errorf("FromCwdOrFallback() at home = %q, want non-empty", got)
+	}
+	if want, _ := FallbackName(); got != want {
+		t.Errorf("FromCwdOrFallback() = %q, want FallbackName() = %q", got, want)
+	}
+}
+
+func TestFromCwdOrFallback_OutsideHomeUsesCwdName(t *testing.T) {
+	t.Chdir(t.TempDir())
+
+	cwd, _ := FromCwd()
+	if cwd == "" {
+		t.Skip("temp dir derived an empty name; cannot assert passthrough")
+	}
+	got, err := FromCwdOrFallback()
+	if err != nil {
+		t.Fatalf("FromCwdOrFallback() error: %v", err)
+	}
+	if got != cwd {
+		t.Errorf("FromCwdOrFallback() = %q, want FromCwd() = %q", got, cwd)
+	}
+}
+
 func TestFromPath_DisambiguatesSanitizedCollisions(t *testing.T) {
 	gotSpace := FromPath("/Users/u/foo bar", "/Users/u")
 	gotUnderscore := FromPath("/Users/u/foo_bar", "/Users/u")