feat: user input bordered separator — enclose prompt in --- style dividers

[VforVitorio]

Context

Currently the user message and model response flow together visually.
Claude Code and similar tools visually separate the user input area with
horizontal rule dividers, making turns easier to scan.

Proposed behaviour

After the user submits a message, echo it inside a visual block before 'thinking':

 ────────────────────────────────────────────────────────
  listame los archivos de mi carpeta src
 ────────────────────────────────────────────────────────
⠋ thinking…  42 tok

Or just a top rule + indented message with no bottom rule (cleaner).

Implementation sketch

In Agent.run(), after receiving user_input and before the Live spinner:

width = console.width
console.print(f"[{TEXT_MUTED}] {'─' * (width - 2)}[/]")
console.print(f"  {user_input}")
console.print(f"[{TEXT_MUTED}] {'─' * (width - 2)}[/]\n")

[VforVitorio]

Implemented in feat/ui-status-bar. A styled separator rule is printed before the thinking spinner starts, visually enclosing the user input and making the boundary between turns clear.