feat(coding-agent): implement new compaction system with overflow recovery

Phase 1: Updated compaction.ts
- findCutPoint now returns CutPointResult with isSplitTurn and turnStartIndex
- Can cut at user, assistant, or bashExecution messages (never tool results)
- Added turnPrefixSummary support for split turns (parallel summarization)
- estimateTokens helper for context size estimation

Phase 2: Updated session-manager.ts
- CompactionEntry now has optional turnPrefixSummary field
- loadSessionFromEntries injects both summaries when turn was split

Phase 3: Updated agent-session.ts
- Overflow detection via isContextOverflow after agent_end
- Proactive compaction check on turn_end before next LLM call
- _abortingForCompaction flag to skip saving aborted messages
- Auto-retry after overflow recovery or proactive compaction
- New event fields: reason (overflow/threshold), willRetry

Phase 4: Updated interactive-mode.ts
- Shows reason in compaction status (Context overflow detected...)
- Shows retry status after compaction

Tests updated for new CutPointResult return type.

packages/coding-agent/src/modes/interactive/interactive-mode.ts

@@ -560,25 +560,27 @@ export class InteractiveMode {
 				this.ui.requestRender();
 				break;
 
-			case "auto_compaction_start":
+			case "auto_compaction_start": {
 				// Set up escape to abort auto-compaction
 				this.autoCompactionEscapeHandler = this.editor.onEscape;
 				this.editor.onEscape = () => {
 					this.session.abortCompaction();
 				};
-				// Show compacting indicator
+				// Show compacting indicator with reason
 				this.statusContainer.clear();
+				const reasonText = event.reason === "overflow" ? "Context overflow detected, " : "";
 				this.autoCompactionLoader = new Loader(
 					this.ui,
 					(spinner) => theme.fg("accent", spinner),
 					(text) => theme.fg("muted", text),
-					"Auto-compacting... (esc to cancel)",
+					`${reasonText}Auto-compacting... (esc to cancel)`,
 				);
 				this.statusContainer.addChild(this.autoCompactionLoader);
 				this.ui.requestRender();
 				break;
+			}
 
-			case "auto_compaction_end":
+			case "auto_compaction_end": {
 				// Restore escape handler
 				if (this.autoCompactionEscapeHandler) {
 					this.editor.onEscape = this.autoCompactionEscapeHandler;
@@ -602,9 +604,14 @@ export class InteractiveMode {
 					compactionComponent.setExpanded(this.toolOutputExpanded);
 					this.chatContainer.addChild(compactionComponent);
 					this.footer.updateState(this.session.state);
+
+					if (event.willRetry) {
+						this.showStatus("Compacted context, retrying...");
+					}
 				}
 				this.ui.requestRender();
 				break;
+			}
 		}
 	}