Add optional stateless MCP server for the real browser

Expose a conservative browser-cli tool surface to MCP hosts without
duplicating browser-control logic: every tool is a thin adapter over the
existing Python SDK, and each call builds a fresh BrowserCLI so the real
browser stays the only owner of tab and navigation state.

The MCP process resolves BROWSER_CLI_PROFILE, BROWSER_CLI_REMOTE, and
BROWSER_CLI_KEY explicitly instead of passing None down, so a pinned server
targets one browser rather than fanning listing calls out across every
connected browser. Explicit tool arguments still win.

Tool names keep a browser_ prefix for hosts that expose raw MCP names, while
BROWSER_CLI_MCP_TOOL_PREFIX lets hosts that already namespace by server drop
it and avoid names like browser_cli_testing_browser_tabs_list.

JavaScript evaluation, raw commands, storage writes, and session import stay
unexposed, and Streamable HTTP refuses non-loopback binds because the endpoint
has no authentication of its own; remote browsers go through browser-cli's
authenticated remote transport instead.

MCP stays an optional extra, so normal installs are unaffected.

browser_cli/mcp/naming.py

@@ -0,0 +1,39 @@
+"""Tool-name prefixing for the MCP surface.
+
+Hosts differ in how they namespace MCP tools. Hosts that already prefix tool
+names with the server name turn the built-in ``browser_`` prefix into stutter
+(``browser_cli_testing_browser_tabs_list``), while hosts that expose raw names
+need the prefix to keep ``navigate`` or ``screenshot`` unambiguous. The prefix
+is therefore configurable per MCP server process.
+"""
+from __future__ import annotations
+
+import os
+import re
+
+TOOL_PREFIX_ENV = "BROWSER_CLI_MCP_TOOL_PREFIX"
+DEFAULT_TOOL_PREFIX = "browser_"
+
+_VALID_PREFIX = re.compile(r"\A[a-z][a-z0-9_]*\Z")
+
+def resolve_tool_prefix(environ: dict[str, str] | None = None) -> str:
+  """Return the configured tool-name prefix, defaulting to ``browser_``.
+
+  An empty value disables prefixing for hosts that namespace tools themselves.
+  """
+  configured = (environ if environ is not None else os.environ).get(TOOL_PREFIX_ENV)
+  if configured is None:
+    return DEFAULT_TOOL_PREFIX
+  prefix = configured.strip()
+  if not prefix:
+    return ""
+  if not _VALID_PREFIX.match(prefix):
+    raise ValueError(
+      f"{TOOL_PREFIX_ENV} must be lowercase letters, digits, and underscores starting "
+      f"with a letter, or empty to disable prefixing; got {configured!r}"
+    )
+  return prefix if prefix.endswith("_") else f"{prefix}_"
+
+def tool_name(base: str, prefix: str) -> str:
+  """Apply *prefix* to a bare tool name such as ``tabs_list``."""
+  return f"{prefix}{base}"