agent: mid-turn compaction (a single turn that grows past the budget) #7

Follow-up to issue 939. The SummarizingCompactor fires once, at the top of a turn, on the incoming history (via RunnerConfig.Compactor). That covers the common case (a long conversation), but not a single turn that itself balloons past the context budget mid-loop — e.g. many parallel tool calls returning large results within one turn.

Scope

• Re-check the estimate inside the Runner loop (between steps) and re-run the Compactor on the working msgs when it crosses the budget, emitting another EventCompaction.
• Decide the interaction with tool-result offloading (966): offloading already shrinks large tool outputs to stubs, so mid-turn compaction is the fallback for when even the stubs + assistant text exceed budget.
• Guard against a compaction/So-still-too-big loop (a hard floor; abort the turn with a clear error if a single step can't fit).

Roadmap notes this as the "L for mid-turn" effort. The pre-loop hook shipped in 939 is the "M" version.