feat(cli): bare agent opens a connected session

[hbrooks]

What

Any agent invocation that isn't a known subcommand or a top-level --help/--version is now shorthand for agent session start --connect ..., forwarding the prompt and every trailing flag into a fresh connected session:

• agent → opens a connected session
• agent "fix the tests" → agent session start --connect "fix the tests"
• agent "fix the tests" --model claude-opus-4-8 → prompt + all flags forwarded
• agent --model claude-opus-4-8 "fix" → leading flags forwarded too

How

src/cli.tsx inspects the first arg before parsing. If it's a known subcommand (or an alias), or a top-level -h/--help/-V/--version/help, it dispatches unchanged. Otherwise it splices session start --connect in front of the args:

const topLevelCommands = new Set([
  'help',
  ...program.commands.flatMap((c) => [c.name(), ...c.aliases()]),
])
const first = process.argv[2]
const isTopLevel =
  first === '-h' || first === '--help' ||
  first === '-V' || first === '--version' ||
  (first !== undefined && topLevelCommands.has(first))
if (!isTopLevel) {
  process.argv.splice(2, 0, 'session', 'start', '--connect')
}

Verification

bun run typecheck passes, and each dispatch path resolves correctly (checked via --help so nothing spends money):

Invocation	Resolves to
agent --help	top-level help
agent "fix the tests" --help	session start (prompt + flags forwarded)
agent --model x "fix" --help	session start (leading flag forwarded)
agent session list --help	subcommand untouched
agent --version	version string

Caveats

• The prompt must be quoted. agent lets fix the tests (unquoted) becomes four positionals and start takes one [prompt], so Commander errors with "too many arguments". Quoted works.
• Subcommand typos now forward to connect as a prompt rather than erroring with "unknown command".
• Bare/prompt invocations start a new session (spin up a sandbox) by default — inherent to making start --connect the default.