Addresses #22599
## Why
`/side` currently lets `Esc` return to the parent thread. Multiple users
reported that this collides with queued-steer UI that also advertises
`Esc`, so a timing-sensitive keypress can dismiss an ephemeral side chat
instead of sending the queued prompt.
After removing that dismissal shortcut, the same `Esc` path could fall
through to main-thread backtrack/edit-previous handling, which is not
valid for ephemeral side conversations. This keeps `/side` out of both
global `Esc` behaviors.
## What changed
- Remove `Esc` from the `/side` return shortcut matcher while keeping
the existing `Ctrl+C` and `Ctrl+D` behavior.
- Update side-conversation hints and blocked-command copy to advertise
`Ctrl+C` as the return shortcut.
- Rename the reserved `Esc` keymap label to describe backtracking only.
- Block backtrack/edit-previous handling while a side conversation is
active and report `Editing previous prompts is unavailable in side
conversations.` when that path would have fired.
- Keep composer-owned `Esc` behavior, such as Vim insert-mode escape,
routed locally.
- Refresh focused shortcut assertions and TUI snapshots for the updated
footer and new side-conversation error message.
## Verification
Manually tested `/side` use cases and `Esc`, `Ctrl+C`, `Ctrl+D`.
The TUI supports long-running turns and agent threads, but quick side
questions have required interrupting the main flow or manually
forking/navigating threads. This PR adds a guarded `/side` flow so users
can ask brief side-conversation questions in an ephemeral fork while
keeping the primary thread focused. This also helps address the feature
request in #18125.
The implementation creates one side conversation at a time, lets `/side`
open either an empty side thread or immediately submit `/side
<question>`, and returns to the parent with Esc or Ctrl+C. Side
conversations get hidden developer guardrails that treat inherited
history as reference-only and steer the model away from workspace
mutations unless explicitly requested in the side conversation.
The TUI hides most slash commands while side mode is active, leaving
only `/copy`, `/diff`, `/mention`, and `/status` available there.
Eric TrautandGitHub