fix(browser): parse flags after text values and fail on missing sessions

[junhsss]

fill, type, setvalue and set useragent captured every argument after the value, including --session. The command then ran on the default session. If no default daemon was running, the CLI silently started a new cloud session. Agents saw "Unknown ref" or a blank page while their real session sat idle.

• Drop trailing_var_arg from the four value args. Flags now parse anywhere, and unquoted multi-word values still work.
• --session <name> no longer starts a new session when its daemon is not running. It fails and suggests steel browser start --session <name>. This also covers sessions that expired or idled out. Those used to come back silently with default settings.
• Starting the implicit default session prints a warning to stderr, also in JSON mode.