Pause once at a configurable context threshold and accept later requests #4

Add an optional once-per-session pause threshold to no-auto-compaction mode. At the first safe execution boundary after the threshold is reached, end autonomous processing without asking the model to obey an instruction. Use the same active-context estimate and usable-window denominator as the full-window guard.

Set the durable pause marker before reporting the pause. Preserve completed tool results and already accepted input; do not cancel running tools or leave unmatched tool calls in history. A later explicit user message resumes normal processing, including ordinary work or a handoff. A process restart must not re-arm the pause for the same session; a fresh session starts with its own marker. Keep pause state local to the session whose budget crossed the threshold.

Ship integration tests for a below-threshold turn, one threshold crossing, multiple tool results crossing together, a subsequent handoff, a subsequent ordinary request, persisted state, and independent sessions. Assert model-request counts and preserved history so an advisory-only implementation fails the tests.

Done when the runtime behavior is proved without TUI dependencies. Publish a typed pause outcome through the existing turn/event mechanism, keeping any required protocol changes backward compatible. Presentation is the next issue.