fix(code): keep /update and /install --package prompts responsive

[Mason Daugherty (mdrxy)]

Related #5086

Three confirmation prompts froze the terminal instead of taking an answer: the /update dependency-refresh prompt, the /update --deps app-update prompt, and the /install <pkg> --package prompt. Enter and Esc did nothing, the quit key did nothing, and the only way out was killing the terminal. They work now.

Why it happened

Textual delivers keypresses on a single loop, and slash commands are handled on that same loop. These three commands sat there waiting for the user to answer the prompt — so the loop was blocked waiting for a keypress that it was itself responsible for delivering. The prompt could never resolve. Its internal timeout is ten minutes, so the app looked permanently dead rather than slow.

#5086 fixed this same deadlock for the post-install restart prompt. These three prompts were missed.

The fix

_schedule_off_message_pump runs the prompt, and whatever depends on the answer, as a separate task. The command handler returns right away, the loop is free, and keypresses reach the prompt. Two blocks of code moved so they could be reached from the new task: _handle_update_command's upgrade tail became _perform_app_upgrade, and the --package install body became _perform_package_install.

What that broke

Blocking the loop was also doing two useful things by accident. Both are now done deliberately, and this is most of the diff.

It prevented two installs at once. /install and /update both run uv tool install --reinstall against the same environment, and each rebuilds that environment from what is already recorded in it. Run two at once and both read the old state first, so whichever finishes last silently discards the other's package. Only one install or update can now be in progress at a time; a second command waits in the queue rather than starting alongside.

It kept failures visible. Code in a separate task is outside the command handler's error handling, so a crash would only reach a log file the user never sees — leaving Installing package '<pkg>'... on screen as the last word, with nothing actually running. Each prompt now reports its own failures on screen.

There is also a new case to handle: quitting while an install is running cancels it partway through uv, which can leave the environment half-updated. That now gets written to the log, and the install is given a moment to finish cleaning up before the app exits so it cannot leave a stray process behind.

Package-name validation and the editable-install refusal still run before the hand-off, so the prompt remains the only gate on installing third-party code.

Tests

Each of the three prompts gets a test that submits the command the way the input widget does, then presses a key. Against the old code the keypress never arrives and the prompt never resolves, so each test fails.

libs/code/AGENTS.md already warned against waiting on a modal inside a slash-command handler. It now names the helper to use instead, and what a caller owes it: report your own errors, expect to be cancelled partway, and take the lock before touching the environment.

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