[jiangchuanso]

Summary

Introduce reactive compaction: instead of compacting only on a fixed trigger, the agent reacts to context-window pressure, preserves the most recent original messages, and emits context/compaction events so hosts (e.g. JSON stream consumers) can observe what happened.

• Adaptive reservation + window cap (crates/aion-config/src/compact.rs, config.rs): compaction reserve and a configurable window cap are parsed from config, so reserved headroom adapts to the effective context window.
• Reactive flow (crates/aion-agent/src/compact/auto.rs, estimate.rs, context_usage.rs, engine.rs): compaction is driven by context-usage signals; recent original turns are kept verbatim instead of being folded into the summary. New context/compaction notifications are published through the output sinks (output/sink.rs, output/protocol_sink.rs).
• Window-trimmed summarization input, and responses exceeding the window (HTTP 400) are now classified as PromptTooLong rather than a generic error (crates/aion-providers/src/error.rs, transport.rs, retry.rs).
• Protocol events (crates/aion-protocol/src/events.rs, commands.rs, crates/aion-types/src/compact.rs): new context/compaction events are added to the JSON stream protocol and documented in docs/json-stream-protocol.md.

Test plan

• Unit tests cover the auto-compact policy (auto_test.rs), provider error classification (error_test.rs) and protocol command construction (commands_test.rs).
• Integration test crates/aion-agent/tests/autocompact_test.rs exercises the end-to-end compact behavior.
• Existing cargo test / cargo clippy gates in CI validate the change.