[FEATURE]: Show prompt after /compact to choose continue or branch #17802

Problem

When users run /compact to summarize context, there's no feedback after compaction completes - users are left wondering what happened and how to continue.

Proposed Solution

After manual compaction, show an inline prompt (similar to permission/question prompts) with options:

• Continue - Keep working in the current session
• New Session - Start a new session with the compaction summary as context

Auto compaction should continue silently without interruption.

Similar Work

• PR feat: add /branch command to create new session with compacted context #7824 adds /branch command for creating new sessions with compacted context - this is complementary to that feature