Render postToolUse hook messages in the Copilot desktop app #5374

Problem

postToolUse hooks can return output through modifiedResult and additionalContext, but the Copilot desktop app does not render the modified result in the user-facing tool-result surface.

A hook may execute successfully and return a message intended for the user, optionally alongside additional context intended for the model. In the desktop app, that message is currently surfaced only as "Additional guidance from postToolUse hooks" rather than in the normal tool result or in a dedicated user-facing message box. This makes hook-generated information easy to miss and differs from the GitHub Copilot CLI timeline presentation.

Expected behavior

The desktop app should provide a user-visible surface for a message returned by a postToolUse hook, while preserving additionalContext for the model. The exact presentation is an implementation choice; the essential requirement is that hook-returned user messages are clearly visible in the conversation.

Relation

Relates to #5099 (display-only hook for assistant messages).