feat(mecatui): move the session status line above the input box

Problem

The session status indicator — "Thinking", "Running Bash", etc. — renders in the gutter below the chat input box. Users instinctively look above the input area for signs that the agent is doing something, so the current placement is easy to miss. The bottom gutter doesn't feel prominent enough for what is the primary "is anything happening?" signal in the TUI.

Suggestion

Move the status line so it sits directly above the input area, between the transcript and the prompt. That puts activity feedback in the user's natural line of sight as they finish typing and wait for a response.

Notes

• Worth considering alongside #1217 (showing the active session title in stock status lines) since both touch the same status rendering.
• Reported by @dussab (screenshot included in the original report).

Acceptance

• Status text ("Thinking", tool-running states, etc.) appears immediately above the input box.
• The bottom gutter either drops the status or keeps only secondary info (e.g. hints/keybindings) without duplicating it.