🤖 fix: treat default Continue as resume sentinel

[ammar-agent]

What changed

• Use a short resume sentinel ("Continue") for forced compaction.
• Don't inject that sentinel into the compaction prompt (summary request), while still keeping it in compaction metadata so the backend can auto-send it after compaction.
• Add unit coverage for the default vs. explicit follow-up behavior.

Why

Appending a verbose auto-injected instruction can compete with the user's actual message; treating "Continue" as a control sentinel and keeping it out of the summarization prompt reduces instruction confusion.

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