Simplify compaction: remove proactive abort, use Agent.continue() for retry

- Add agentLoopContinue() to pi-ai for resuming from existing context
- Add Agent.continue() method and transport.continue() interface
- Simplify AgentSession compaction to two cases: overflow (auto-retry) and threshold (no retry)
- Remove proactive mid-turn compaction abort
- Merge turn prefix summary into main summary
- Add isCompacting property to AgentSession and RPC state
- Block input during compaction in interactive mode
- Show compaction count on session resume
- Rename RPC.md to rpc.md for consistency

Related to #128

packages/coding-agent/docs/rpc.md

@@ -109,6 +109,7 @@ Response:
     "model": {...},
     "thinkingLevel": "medium",
     "isStreaming": false,
+    "isCompacting": false,
     "queueMode": "all",
     "sessionFile": "/path/to/session.jsonl",
     "sessionId": "abc123",