Let me edit the agent's proposed content inline before authorizing the action

## Request

Let me edit the agent's proposed content inline in the chat, then authorize it — instead of spending a full turn describing a tiny fix.

## Example

I ask: **"Update my GitHub profile."** The agent replies with proposed bio text that's almost perfect, but the last sentence needs to go. Today I have to write a follow-up ("drop the last sentence") and wait for the agent to regenerate. Instead, I want to delete that sentence directly in the agent's message and just say **"make the update"** — and the agent uses my edited version as the input to its action.

## Why this is different from existing requests

- **Not** user-message editing (#1602) — that re-runs my prompt and discards the agent's output.
- **Not** diff editing (#1397) — here the content lives only in the chat message; nothing is on disk yet.

The agent's message is sometimes the deliverable itself. Treat it as an editable draft I can amend before authorizing the action (like editing an AI-drafted email before hitting send).

Start by tracing the chat message and action-authorization flow described in the request. Done means a user can edit proposed agent content inline, authorize the action, and have the action use the edited version rather than the original draft.

Written by the indexing model from the issue text.