fix(client): validate active browser endpoints

- Check Unix socket reachability with a real connection attempt instead of treating any existing path as active.
- Report ambiguous host-only remote aliases with actionable --remote/--browser examples.
- Update client tests to use listening Unix sockets and cover ambiguous remote alias errors.
- Bump package and extension versions to 0.10.2.

browser_cli/client.py

@@ -79,13 +79,24 @@ class BrowserTarget:
     socket_path: str
     remote: str | None = None
 
+def _is_reachable_unix_endpoint(endpoint: str) -> bool:
+    """Return True when a Unix socket path exists and accepts connections."""
+    path = Path(endpoint)
+    if not path.exists():
+        return False
+    try:
+        with socket.socket(socket.AF_UNIX, socket.SOCK_STREAM) as sock:
+            sock.settimeout(0.2)
+            sock.connect(endpoint)
+        return True
+    except OSError:
+        return False
 
 def _active_endpoints(reg: dict) -> dict:
     """Return only entries whose endpoint appears reachable."""
     if is_windows():
         return dict(reg)
-    return {k: v for k, v in reg.items() if Path(v).exists()}
-
+    return {k: v for k, v in reg.items() if _is_reachable_unix_endpoint(v)}
 
 def display_browser_name(profile_name: str, sock_path: str) -> str:
     if profile_name != "default":
@@ -198,6 +209,23 @@ def remote_target_for_alias(alias: str | None) -> BrowserTarget | None:
             endpoint_matches.append(target)
     if len(endpoint_matches) == 1:
         return endpoint_matches[0]
+    if len(endpoint_matches) > 1:
+        aliases = [target.profile for target in endpoint_matches]
+        endpoint = endpoint_matches[0].remote or alias
+        examples = "\n".join(
+            f"  browser-cli --remote {endpoint} --browser {a} ..."
+            for a in aliases
+        )
+        display_aliases = [target.display_name for target in endpoint_matches]
+        shorthand_examples = "\n".join(
+            f"  browser-cli --browser {a} ..."
+            for a in display_aliases
+        )
+        raise BrowserNotConnected(
+            f"Multiple remote browser instances are active on {alias}: {', '.join(aliases)}\n"
+            f"Use --browser <alias> with --remote to select one:\n{examples}\n"
+            f"Or use the full remote browser alias:\n{shorthand_examples}"
+        )
     return None