feat: 實作 bear CLI PAT 模式 MVP（login/whoami/token/logout/status）

- 新增 Elixir + Mix escript 專案（Req 0.5+、Jason）
- login --token <PAT>：以 PAT 驗證 /userinfo 後寫入憑證檔（0600）
- whoami：GET /userinfo 顯示身分 claims（支援 --json）
- token：印出 access token 供 pipe（--refresh 在 PAT 模式忽略）
- logout：清除本機憑證；status：純本機判定
- 憑證檔採原子寫入（rename）、0600、O_CREAT|O_EXCL 防 symlink
- 全域選項：--issuer/--config/--json/-v/-h/--version
- 29 個單元測試；mix precommit（compile --warnings-as-errors + format + test）
- 更新 README.md 與 docs/commands.md 反映 PAT 模式 MVP

Device Flow 登入待伺服器端 P1（bear 倉庫）完成後再接。issue #2

test/bear_cli/credentials_test.exs

@@ -0,0 +1,62 @@
+defmodule BearCli.CredentialsTest do
+  use ExUnit.Case, async: true
+
+  alias BearCli.Credentials
+
+  test "save/load round-trips JSON" do
+    path = tmp_path()
+    on_exit(fn -> File.rm(path) end)
+
+    assert :ok ==
+             Credentials.save(
+               %{"issuer" => "x", "access_token" => "tok", "email" => "a@b.c"},
+               path
+             )
+
+    assert {:ok, %{"access_token" => "tok", "email" => "a@b.c"}} = Credentials.load(path)
+  end
+
+  test "save creates the file with 0600 permissions" do
+    path = tmp_path()
+    on_exit(fn -> File.rm(path) end)
+    :ok = Credentials.save(%{"access_token" => "tok"}, path)
+
+    mode = File.stat!(path).mode
+    assert Bitwise.band(mode, 0o777) == 0o600
+  end
+
+  test "save creates the parent directory" do
+    base = Path.join(System.tmp_dir!(), "bear_cli_nested_#{System.unique_integer([:positive])}")
+    path = Path.join([base, "sub", "credentials.json"])
+    on_exit(fn -> File.rm_rf(base) end)
+
+    :ok = Credentials.save(%{"a" => "b"}, path)
+    assert File.regular?(path)
+  end
+
+  test "load returns :error for a missing file" do
+    assert :error == Credentials.load("/nonexistent/credentials.json")
+  end
+
+  test "load returns :error for corrupt JSON" do
+    path = tmp_path()
+    on_exit(fn -> File.rm(path) end)
+    File.write!(path, "{not json")
+
+    assert :error == Credentials.load(path)
+  end
+
+  test "delete removes the file and is idempotent" do
+    path = tmp_path()
+    on_exit(fn -> File.rm(path) end)
+    :ok = Credentials.save(%{"a" => "b"}, path)
+
+    assert :ok == Credentials.delete(path)
+    assert :error == Credentials.load(path)
+    assert :ok == Credentials.delete(path)
+  end
+
+  defp tmp_path do
+    Path.join(System.tmp_dir!(), "bear_cli_creds_#{System.unique_integer([:positive])}.json")
+  end
+end