Passing --add-dir suppresses the initial positional prompt; the session opens and never starts a turn

Passing `--add-dir` stops a session from submitting its initial positional prompt. The session opens, the prompt sits in the composer, and no turn starts. Removing `--add-dir` and changing nothing else makes the same prompt submit immediately.

This breaks any wrapper that starts a session on a prompt and grants a directory in the same command: the session looks healthy and never does any work.

## Environment

- Claude Code 2.1.259 and 2.1.263, both affected
- Installed as an npm global package
- Linux x86_64, Node 20.19.2

## Steps

With a prompt file at `/prompt.md` holding one line, and the working directory somewhere other than ``:

| # | Command | Result |
|---|---|---|
| 1 | `claude @/prompt.md` | Submits, answers |
| 2 | `claude "reply with the single word ready, then stop"` | Submits, answers |
| 3 | `claude --add-dir @/prompt.md` | Opens, no turn starts |
| 4 | `claude --add-dir "reply with the single word ready, then stop"` | Opens, no turn starts |
| 5 | `claude --bg @/prompt.md` | Starts working |

1 and 3 differ only by `--add-dir`. 2 and 4 differ only by `--add-dir`. The prompt form does not matter: a text argument and an `@` reference behave the same way in each pair.

## Expected

3 and 4 submit their prompt and start a turn, as 1 and 2 do.

## Actual

The session opens and waits indefinitely. Submitting the prompt by hand works normally, so the session itself is healthy.

## Notes

- Not version-specific across the range tested: 2.1.259 and 2.1.263 behave identically, with background auto-updates disabled to hold the version steady.
- An `@` reference outside the working directory is read without `--add-dir`, so the grant is not what makes the prompt reachable in case 1.
- The failure is silent. The process starts, the interface renders, and nothing distinguishes it from a slow first turn, so a calling process sees a healthy child that never works.

Reproduce the two failing commands with --add-dir and compare them with the equivalent commands without it and with --bg. Trace the CLI startup handling for --add-dir alongside the initial positional prompt, then add a regression check if the project’s existing CLI tests cover these commands. Done means both prompt forms submit and begin a turn while the existing cases continue to work.

Scritto dal modello di indicizzazione a partire dal testo della issue.