conversation transport: abort active turn on client disconnect #7

Is your feature request related to a problem?

When a client closes the tab / disconnects mid-turn, the conversation transport keeps the turn running to its full wall budget. POST /api/agent/sessions/:id/abort exists and works cleanly (ABORTED outcome + persisted partial transcript), but nothing wires client disconnect to it — request.raw.on('close') is never hooked to the per-turn AbortController in activeTurns.

Describe the solution you'd like

In the transport's message handler, when the request socket closes before the turn completes, call ac.abort() for that session's active turn (the same clean ABORTED path the explicit route uses). Should be gated by a deps flag (e.g. abortOnDisconnect?: boolean) so hosts can opt out — some deployments proxy long-polls where socket close is routine.

Context

Lionidas consumed this via plan 20261019T224955Z (agent-harness arc, PR #91) — everything else in that arc was host-side; this is the one seam that needs to live inside butler's transport.