feat(cli): configurable busy input mode + fix /queue always working (#3298)

Two changes:

1. Fix /queue command: remove the _agent_running guard that rejected
   /queue after the agent finished. The prompt was deferred in
   _pending_input until the agent completed, then the handler checked
   _agent_running (now False) and rejected it. /queue now always queues
   regardless of timing.

2. Add display.busy_input_mode config (CLI-only):
   - 'interrupt' (default): Enter while busy interrupts the current run
     (preserves existing behavior)
   - 'queue': Enter while busy queues the message for the next turn,
     with a 'Queued for the next turn: ...' confirmation
   Ctrl+C always interrupts regardless of this setting.

Salvaged from PR #3037 by StefanoChiodino. Key differences:
- Default is 'interrupt' (preserves existing behavior) not 'queue'
- No config version bump (unnecessary for new key in existing section)
- Simpler normalization (no alias map)
- /queue fix is simpler: just remove the guard instead of intercepting
  commands during busy state

cli-config.yaml.example

@@ -688,6 +688,12 @@ display:
   # Toggle at runtime with /verbose in the CLI
   tool_progress: all
 
+  # What Enter does when Hermes is already busy in the CLI.
+  #   interrupt: Interrupt the current run and redirect Hermes (default)
+  #   queue:     Queue your message for the next turn
+  # Ctrl+C always interrupts regardless of this setting.
+  busy_input_mode: interrupt
+
   # Background process notifications (gateway/messaging only).
   # Controls how chatty the process watcher is when you use
   # terminal(background=true, check_interval=...) from Telegram/Discord/etc.