[FEATURE]: /handoff — continue a long session in a fresh context, without LLM re-summarization

Feature hasn't been suggested before.

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Describe the enhancement you want to request

Long sessions eventually get noisy enough that the model starts tripping over its own history. /compact helps but keeps you in the same session; forking copies everything. What I keep wanting is a fresh session that starts with just enough to continue the work.

This has come up before — #26757 and #21760 asked for essentially this and both went stale; #40578 implemented it on top of LLM compaction and got caught by the PR cleanup bot before review. My take differs in one way: instead of re-summarizing with an LLM, assemble the handoff deterministically from what's already in the session — recent user messages, the closing notes from each turn, todos, git state. Verbatim, instant, nothing hallucinated.

I have a small prototype of this running locally and it's been surprisingly useful. Happy to share details or a PR if the team thinks this belongs in core.