Move zarvis ready hint to input row

[edwin-zvs]

Why

• New zarvis sessions currently show the onboarding guidance at the top, which separates it from the input row and makes it hard to spot where to type.

What changed

• Remove the long instruction block from the session banner.
• Add a compact inline ready hint in the editor prompt row when idle:
  • type your prompt and press Enter
• Keep prompt sizing and cursor behavior consistent via existing editor pane row/layout logic.
• Added focused editor-pane tests in cli UI module.

Validation

• cargo test -p agentd-cli editor_pane_ -- --nocapture
• cargo test -p agentd-adapter-zarvis --no-run
• cargo build (workspace)