Agents Window: suggest next user message as ghost text in the chat input

## Problem

When an agent response finishes in the Agents Window, the user has to think of and type the next message themselves, even when the natural next step is obvious from what the agent just said (e.g. "Should I go ahead and make these changes?", "Do you want me to also update the tests?", or a clear next action implied by the final response).

Related: #305023 proposed clickable quick replies for this same moment, but was closed for lack of upvotes rather than for being out of scope.

## Proposed UX

- After an agent response completes, propose a natural next user message directly in the chat input as placeholder-style "ghost text" (similar to inline suggestions in the editor).
- The suggestion is generated from the agent's final response (MVP), not the full session history.
- It only appears when the input is empty and focused, and disappears as soon as the user starts typing.
- Accept it with `Tab`, the same way inline completions are accepted in the editor - this inserts it as normal editable text, it does not auto-send.
- No suggestion is shown when the input already has other content coming from the agent (e.g. an explicit follow-up/next question), so it never competes with existing "what's next" affordances.

This keeps the interaction quiet at rest and out of the way, while giving a fast path for the common case where the next message is predictable.

## Non-goals (for an initial version)

- Multiple/cyclable suggestions.
- Suggestions outside the Agents Window.
- Auto-sending the suggestion.
- Persisting the suggestion or synchronizing it across clients.