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/src/modes/rpc/rpc-mode.ts

@@ -90,6 +90,7 @@ export async function runRpcMode(session: AgentSession): Promise<never> {
 					model: session.model,
 					thinkingLevel: session.thinkingLevel,
 					isStreaming: session.isStreaming,
+					isCompacting: session.isCompacting,
 					queueMode: session.queueMode,
 					sessionFile: session.sessionFile,
 					sessionId: session.sessionId,

packages/coding-agent/src/modes/rpc/rpc-types.ts

@@ -63,6 +63,7 @@ export interface RpcSessionState {
 	model: Model<any> | null;
 	thinkingLevel: ThinkingLevel;
 	isStreaming: boolean;
+	isCompacting: boolean;
 	queueMode: "all" | "one-at-a-time";
 	sessionFile: string;
 	sessionId: string;