Trigger auto-compaction when context occupancy crosses threshold

[EightRice]

Goal

When an agent's running context approaches its provider's window, automatically run the SDK compaction step instead of failing or silently exceeding.

Current state

atn/providers/codex_bridge.py and the bridge code track last_round_input_tokens and have context_window from model_specs.py. The data is there to compute occupancy. No agent currently consumes it to trigger compaction.

Scope

• Add an occupancy check after each turn in the runtime loop.
• When occupancy > N% of context_window(model) (suggested default: 70%), trigger the SDK's compact action before the next turn starts.
• Make the threshold configurable per agent, with a sensible default.
• Emit a runtime event so the snapshot/UI can show "compacted at turn X."

Acceptance

• A long-running agent passes through the threshold and continues without hitting the provider's hard limit.
• Snapshot reflects the compaction event.