Add !attach support to interactive chat

[simonw]

!attach url-or-path in chat.

Codex PR

## Summary

• add !attach <url-or-path> to interactive chat sessions
• support attachments alongside !multi and !fragment
• use the shared chat loop, making the command available to llm chat and llm openai endpoint --chat
• document the command and test attachment delivery and logging

Why

Interactive chat sessions had no command for attaching a file or URL to a later turn. This implements the same general feature proposed in #734, updated for the current shared chat loop and message logging architecture.

Validation

• uv run pytest -q tests/test_chat.py tests/test_openai_endpoint.py — 37 passed
• uv run black --check llm/cli.py tests/test_chat.py
• uv run ruff check llm/cli.py tests/test_chat.py

[simonw]

Doesn't feel quite right. It sends the attachment with an empty prompt as soon as you hit enter, so you don't get a chance to say what you want to do with it:

% uv run llm chat
Chatting with gpt-5.6-luna
Type 'exit' or 'quit' to exit
Type '!multi' to enter multiple lines, then '!end' to finish
Type '!edit' to open your default editor and modify the prompt
Type '!fragment <my_fragment> [<another_fragment> ...]' to insert one or more fragments
Type '!attach <url-or-path>' to attach a file
> !attach https://static.simonwillison.net/static/2026/hy3-pelican.png
 
A cartoon stork riding a red bicycle against a light blue background. The bicycle has black wheels, orange pedals, and the stork’s long neck extends upward to its white head and large orange beak.

Related issue: if the URL is invalid it quits the whole chat session (idea here was what if you put a prompt after the attach, but that doesn't make sense because a filename could have spaces in it):

> !attach https://static.simonwillison.net/static/2026/hy3-pelican.png write a haiku about it
Error: Client error '404 Not Found' for url 'https://static.simonwillison.net/static/2026/hy3-pelican.png%20write%20a%20haiku%20about%20it'
For more information check: https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/404

[simonw]

Fixed in 8040274.

• A standalone !attach now queues the attachment and returns to the prompt instead of sending an empty model request.
• Multiple standalone attachments accumulate and are consumed by the next real message only.
• Invalid paths or URLs print an error without terminating the chat or discarding attachments already queued.
• Pending command-line fragments and attachments now also survive entering !multi.

The regression test covers queueing multiple files, continuing after an invalid attachment, attaching them to the next prompt, and not leaking them into the following turn.