feat: 實作 bear CLI 骨架與 PAT 模式共用基礎（issue #6）

- 專案骨架：Elixir + Mix escript（單檔可執行 bear），HTTP 一律 Req
- 設定／憑證檔讀寫：0600、原子寫入（rename）、O_CREAT|O_EXCL 防 symlink
- PAT Bearer 呼叫 /userinfo；--json 輸出；規格化退出碼
- 補齊 main 規格承諾的 BEAR_TOKEN 環境變數（優先於憑證檔、不寫入）
- login 無效 PAT 退出碼 3（依 docs/commands.md §3.1）；token --refresh 用法錯誤 2（§3.3）
- 35 個單元測試；mix precommit（compile --warnings-as-errors + format + test）全綠
- 沿用已關閉 PR #3 的實作（feat/pat-mode-mvp 分支），文件改依 main 現行版本更新

apps 管理指令待 #5 規格與 alterminal/bear#28 API 就緒後實作。

test/bear_cli/config_test.exs

@@ -0,0 +1,63 @@
+defmodule BearCli.ConfigTest do
+  use ExUnit.Case, async: false
+
+  alias BearCli.Config
+
+  @nonexistent "/nonexistent/bear_cli_config.json"
+
+  setup do
+    old_issuer = System.get_env("BEAR_ISSUER")
+    old_config = System.get_env("BEAR_CONFIG")
+
+    on_exit(fn ->
+      restore_env("BEAR_ISSUER", old_issuer)
+      restore_env("BEAR_CONFIG", old_config)
+    end)
+
+    System.delete_env("BEAR_ISSUER")
+    System.put_env("BEAR_CONFIG", @nonexistent)
+
+    :ok
+  end
+
+  test "defaults to the built-in issuer when nothing else is set" do
+    assert Config.resolve_issuer(nil) == "https://alterminal.com"
+  end
+
+  test "CLI flag takes precedence over everything" do
+    System.put_env("BEAR_ISSUER", "https://env.example.com")
+    assert Config.resolve_issuer("https://flag.example.com") == "https://flag.example.com"
+  end
+
+  test "BEAR_ISSUER env wins over config and default" do
+    System.put_env("BEAR_ISSUER", "https://env.example.com")
+    assert Config.resolve_issuer(nil) == "https://env.example.com"
+  end
+
+  test "config file issuer wins over default" do
+    path = write_config(%{"issuer" => "https://config.example.com"})
+
+    on_exit(fn -> File.rm(path) end)
+
+    assert Config.resolve_issuer(nil, path) == "https://config.example.com"
+  end
+
+  test "config path override is honored" do
+    path = write_config(%{"issuer" => "https://override.example.com"})
+
+    on_exit(fn -> File.rm(path) end)
+
+    assert Config.resolve_issuer(nil, path) == "https://override.example.com"
+  end
+
+  defp write_config(map) do
+    path =
+      Path.join(System.tmp_dir!(), "bear_cli_config_#{System.unique_integer([:positive])}.json")
+
+    File.write!(path, Jason.encode!(map))
+    path
+  end
+
+  defp restore_env(key, nil), do: System.delete_env(key)
+  defp restore_env(key, value), do: System.put_env(key, value)
+end