Invalid LIBTMUX_SAFETY values fail open to mutating

[tony]

Problem

Invalid LIBTMUX_SAFETY values currently fall back to mutating. README also documents invalid values: read, read+send, and read+send+kill.

This can expose write tools when a user intends to restrict the server.

Minimal local reproduction

$ LIBTMUX_SAFETY=read uv run python - <<'PY'
import asyncio
from libtmux_mcp.server import build_mcp_server

async def main():
    tools = await build_mcp_server().list_tools()
    names = {tool.name for tool in tools}
    print("send_keys visible:", "send_keys" in names)
    print("kill_pane visible:", "kill_pane" in names)

asyncio.run(main())
PY

Expected today: send_keys visible: True.

Minimal fix

Fail closed.

Either:

• refuse startup on invalid LIBTMUX_SAFETY, or
• fall back to readonly.

Also fix README/server instructions to document only the actual values:

• readonly
• mutating
• destructive

Acceptance criteria

• LIBTMUX_SAFETY=read does not expose mutating tools.
• Invalid safety values produce a clear error or readonly-only surface.
• README, configuration docs, and server instructions use the same safety value names.
• Tests cover invalid safety configuration.