## Why
Users have asked to queue follow-up slash commands while a task is
running, including in #14081, #14588, #14286, and #13779. The previous
TUI behavior validated slash commands immediately, so commands that are
only meaningful once the current turn is idle could not be queued
consistently.
The queue should preserve what the user typed and defer command parsing
until the item is actually dispatched. This also gives `/fast`, `/review
...`, `/rename ...`, `/model`, `/permissions`, and similar slash
workflows the same FIFO behavior as plain queued prompts.
## What Changed
- Added a queued-input action enum so queued items can be dispatched as
plain prompts, slash commands, or user shell commands.
- Changed `Tab` queueing to accept slash-led prompts without validating
them up front, then parse and dispatch them when dequeued.
- Added `!` shell-command queueing for `Tab` while a task is running,
while preserving existing `Enter` behavior for immediate shell
execution.
- Moved queued slash dispatch through shared slash-command parsing so
inline commands, unavailable commands, unknown commands, and local
config commands report at dequeue time.
- Continued queue draining after local-only actions and after slash menu
cancellation or selection when no task is running.
- Preserved slash-popup completion behavior so `/mo<Tab>` completes to
`/model ` instead of queueing the prefix.
- Updated pending-input preview snapshots to show queued follow-up
inputs.
## Verification
I did a bunch of manual validation (and found and fixed a few bugs along
the way).
917a85b0d6