feat: add option to start cli in interactive mode with arg as prompt

[tode-rl]

Add ability to pass a prompt as a command-line argument that executes on entering interactive mode.

Usage:
deepagents "create hello.py"
deepagents --agent mybot "review the last commit"
deepagents --auto-approve "add a new unit test"

Changes:

• parse_args(): Detect if first positional arg is a prompt vs subcommand
• simple_cli(): Accept and execute initial_prompt parameter
• main(): Pass initial_prompt through
• cli_main(): Extract prompt from args and pass to main()
• ui.py: Update help text with [PROMPT] usage and example
• README.md: Document command-line prompt execution feature

[vtrivedy]

looks good with added flags, but instead of entering interactive mode it should exit after it's done

[tode-rl]

looks good with added flags, but instead of entering interactive mode it should exit after it's done

I think it can do that but I believe it should have a separate flag for this behaviour. My thought is that a larger task would benefit from having the context in the cli and the ability to do follow-ups

for a usecase like deepagents --auto-approve "Write a new unit test case" some flag like --script could exit from the loop on completion? (I've seen exec, and -p used in other clis to toggle this behaviour)

[tode-rl]

@vtrivedy Any thoughts on #284 (comment)? I can do in a follow-up if you're ok with adding another arg in the cli. I don't mind the cli exiting after complete but would like some control over it

[vtrivedy]

agree on separate flag, we're leaning towards having the "exec" style only right now (shell exits) to bootstrap the run. if you want to make a new PR we can review it today