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/core/session-manager.ts

@@ -50,8 +50,6 @@ export interface CompactionEntry {
 	type: "compaction";
 	timestamp: string;
 	summary: string;
-	/** Summary of turn prefix when a turn was split (user message to first kept message) */
-	turnPrefixSummary?: string;
 	firstKeptEntryIndex: number; // Index into session entries where we start keeping
 	tokensBefore: number;
 }
@@ -180,18 +178,9 @@ export function loadSessionFromEntries(entries: SessionEntry[]): LoadedSession {
 		}
 	}
 
-	// Build final messages: summaries + kept messages
+	// Build final messages: summary + kept messages
 	const messages: AppMessage[] = [];
-
-	// Add history summary
 	messages.push(createSummaryMessage(compactionEvent.summary));
-
-	// Add turn prefix summary if present (when a turn was split)
-	if (compactionEvent.turnPrefixSummary) {
-		messages.push(createSummaryMessage(compactionEvent.turnPrefixSummary));
-	}
-
-	// Add kept messages
 	messages.push(...keptMessages);
 
 	return { messages, thinkingLevel, model };