fix(review): four findings from adversarial Codex review round 1

- twitter: align backend selection with the two-phase findings pattern —
  an unauthenticated twitter-cli (warn) no longer blocks a fully-working
  OpenCLI (ok) further down the candidate list
- watch: use _is_newer_version like check-update (the != comparison kept
  the downgrade prompt this branch fixed elsewhere)
- uninstall: third copy of the skill-dir removal also gets the symlink
  guard (full `agent-reach uninstall` path)
- doctor: a stale active_backend from a previous check on the singleton
  channel no longer leaks into an errored result

+4 regression tests (162 total).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/test_twitter_channel.py

@@ -152,3 +152,33 @@ def test_check_twitter_cli_broken_falls_back_to_bird():
     assert status == "ok"
     assert "bird" in message
     assert channel.active_backend == "bird CLI (legacy)"
+
+
+def test_unauthenticated_twitter_cli_does_not_block_working_opencli():
+    """warn 候选不得屏蔽排在后面的 ok 候选(Codex review 发现)。"""
+    channel = TwitterChannel()
+    with patch.object(
+        TwitterChannel, "_check_twitter_cli",
+        return_value=("warn", "twitter-cli 已安装但未认证"),
+    ), patch.object(
+        TwitterChannel, "_check_opencli",
+        return_value=("ok", "OpenCLI 可用（复用浏览器登录态）"),
+    ), patch.object(TwitterChannel, "_check_bird", return_value=None):
+        status, msg = channel.check()
+    assert status == "ok"
+    assert channel.active_backend == "OpenCLI"
+
+
+def test_all_warn_falls_back_to_first_warn():
+    channel = TwitterChannel()
+    with patch.object(
+        TwitterChannel, "_check_twitter_cli",
+        return_value=("warn", "twitter-cli 未认证"),
+    ), patch.object(
+        TwitterChannel, "_check_opencli",
+        return_value=("warn", "扩展未连接"),
+    ), patch.object(TwitterChannel, "_check_bird", return_value=None):
+        status, msg = channel.check()
+    assert status == "warn"
+    assert channel.active_backend == "twitter-cli"
+    assert "未认证" in msg