fix(tui): #3032 residuals — running-exec hint now says Ctrl+B backgrounds the command; Ctrl+B documented in KEYBINDINGS.md and runbook updated for menu removal; Cannot-background message names the reason (interactive / non-shell tool / nothing running)

Co-Authored-By: Claude <noreply@anthropic.com>
https://claude.ai/code/session_018zaP8vUfTAsrE38L6h6fw5

docs/KEYBINDINGS.md

@@ -11,6 +11,7 @@ Bindings are not (yet) user-configurable — tracked for a future release (#436,
 | `F1` or `Ctrl-/`     | Toggle the help overlay                                       |
 | `Ctrl-K`             | Open the command palette (slash-command finder)                |
 | `Ctrl-C`             | Cancel current turn / dismiss modal / arm-then-confirm quit    |
+| `Ctrl-B`             | Background the running foreground shell command (turn continues; the command becomes a `/jobs` background job) |
 | `Ctrl-D`             | Quit (only when the composer is empty)                         |
 | `Tab`                | Cycle TUI mode: Plan → Agent → YOLO → Plan                     |
 | `Shift-Tab`          | Cycle reasoning effort: off → high → max → off                 |

docs/OPERATIONS_RUNBOOK.md

@@ -28,7 +28,7 @@ Checks:
 3. Confirm no local sandbox/permission deadlock in tool output
 
 Actions:
-1. If a foreground shell command is running, press `Ctrl+B` and choose whether to background it or cancel the current turn.
+1. If a foreground shell command is running, press `Ctrl+B` to move it to the background (the turn keeps running and the command becomes a background job under `/jobs`); use `Ctrl+C` instead if you want to cancel the turn.
 2. If the command was started in the background, ask the assistant to cancel it with `exec_shell_cancel` and the returned task id.
 3. Use `Esc` or `Ctrl+C` to interrupt the current turn when you want to stop the request itself.
 4. Retry prompt; if still failing, restart TUI.