fix: propagate key through remote discovery; auto-persist key per remote

- remote_browser_targets(), _auto_route_remote(), active_browser_targets()
  now accept and forward the key parameter so pubkey auth works during
  the initial browser-cli.targets discovery call
- _multi_browser_targets() in tabs/groups/windows/session commands now
  reads key from ctx.obj and passes it through
- send_command() auto-saves the key spec (e.g. "agent") to remotes.json
  on first explicit use; subsequent calls to the same remote reuse it
  without requiring --key every time
- Added save_remote_key() / key_for_remote() helpers (mirrors token helpers)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_client.py

@@ -9,6 +9,7 @@ from browser_cli.client import (
     _resolve_socket,
     active_browser_targets,
     display_browser_name,
+    key_for_remote,
     save_remote_token,
     send_command,
     remote_target_for_alias,
@@ -115,7 +116,7 @@ def test_send_command_auto_routes_single_remote_target(monkeypatch):
 
     monkeypatch.setattr(
         "browser_cli.client.remote_browser_targets",
-        lambda endpoint, token=None: [BrowserTarget("work", "host:work", "", remote=endpoint, token=token)],
+        lambda endpoint, token=None, key=None: [BrowserTarget("work", "host:work", "", remote=endpoint, token=token)],
     )
 
     def fake_send_remote(endpoint, msg, private_key=None):
@@ -223,7 +224,7 @@ def test_send_command_requires_browser_for_multiple_remote_targets(monkeypatch):
     monkeypatch.delenv("BROWSER_CLI_PROFILE", raising=False)
     monkeypatch.setattr(
         "browser_cli.client.remote_browser_targets",
-        lambda endpoint, token=None: [
+        lambda endpoint, token=None, key=None: [
             BrowserTarget("main", "host:main", "", remote=endpoint, token=token),
             BrowserTarget("furry", "host:furry", "", remote=endpoint, token=token),
         ],
@@ -255,3 +256,46 @@ def test_active_browser_targets_includes_remote_targets(monkeypatch, tmp_path):
     assert targets[0].display_name == "browser-host.example:work"
     assert targets[0].remote == endpoint
     assert targets[0].token == "secret-token"
+
+
+def test_send_command_auto_saves_and_reuses_key_for_remote(monkeypatch, tmp_path):
+    """--key agent is saved on first use; omitting --key on subsequent calls reuses it."""
+    import json as _json
+
+    remotes_path = tmp_path / "remotes.json"
+    remotes_path.write_text("{}", encoding="utf-8")
+    monkeypatch.setattr("browser_cli.client.REMOTE_REGISTRY_PATH", remotes_path)
+    monkeypatch.setattr("browser_cli.client.REGISTRY_PATH", tmp_path / "missing-registry.json")
+    monkeypatch.delenv("BROWSER_CLI_PROFILE", raising=False)
+    monkeypatch.delenv("BROWSER_CLI_REMOTE", raising=False)
+    monkeypatch.delenv("BROWSER_CLI_TOKEN", raising=False)
+    monkeypatch.delenv("BROWSER_CLI_KEY", raising=False)
+
+    from pathlib import Path as _Path
+    used_keys = []
+
+    def fake_load_private_key(key_path=None):
+        used_keys.append(str(key_path) if key_path is not None else None)
+        return None  # no actual key needed for this test
+
+    monkeypatch.setattr("browser_cli.client._load_private_key", fake_load_private_key)
+    monkeypatch.setattr(
+        "browser_cli.client.remote_browser_targets",
+        lambda endpoint, token=None, key=None: [BrowserTarget("default", "host:default", "", remote=endpoint)],
+    )
+
+    def fake_send_remote(endpoint, msg, private_key=None):
+        return _json.dumps({"success": True, "data": "ok"}).encode()
+
+    monkeypatch.setattr("browser_cli.client._send_remote", fake_send_remote)
+
+    # First call with explicit --key agent
+    send_command("tabs.list", remote="host:8765", key=_Path("agent"))
+    assert used_keys[-1] == "agent"
+
+    # Key must be persisted now
+    assert key_for_remote("host:8765") == "agent"
+
+    # Second call without --key — should reuse saved "agent"
+    send_command("tabs.list", remote="host:8765")
+    assert used_keys[-1] == "agent"