fix(coding-agent): stop anchoring auto-continue prompt on stale 'Next Steps'

The autoContinue post-compaction prompt echoed the summary's '## Next
Steps' heading, but that section is generated only from the compacted
tail; the kept ~20k recent tokens are not fed to the summarizer. When
the user pivoted within the kept window, 'Continue if you have next
steps.' anchored the model on the now-outdated plan instead of the
latest intent.

Move the prompt to prompts/system/auto-continue.md (per AGENTS.md
no-inline-prompts rule) and rewrite it to direct the model to re-read
the kept recent messages and follow the user's most recent request,
explicitly allowing it to stop when nothing remains.

Fixes #840

packages/coding-agent/src/prompts/system/auto-continue.md

@@ -0,0 +1 @@
+Resume work on the user's most recent intent. Re-read the kept recent messages above the summary to confirm what the user asked for last; if their latest request supersedes earlier plans recorded in the summary, follow the latest request. If there is nothing left to do, say so briefly instead of inventing further work.

packages/coding-agent/src/session/agent-session.ts

@@ -121,6 +121,7 @@ import {
 import { getCurrentThemeName, theme } from "../modes/theme/theme";
 import type { PlanModeState } from "../plan-mode/state";
 import autoHandoffThresholdFocusPrompt from "../prompts/system/auto-handoff-threshold-focus.md" with { type: "text" };
+import autoContinuePrompt from "../prompts/system/auto-continue.md" with { type: "text" };
 import eagerTodoPrompt from "../prompts/system/eager-todo.md" with { type: "text" };
 import handoffDocumentPrompt from "../prompts/system/handoff-document.md" with { type: "text" };
 import ircIncomingTemplate from "../prompts/system/irc-incoming.md" with { type: "text" };
@@ -1221,11 +1222,11 @@ export class AgentSession {
 			await this.#promptWithMessage(
 				{
 					role: "developer",
-					content: [{ type: "text", text: "Continue if you have next steps." }],
+					content: [{ type: "text", text: autoContinuePrompt }],
 					attribution: "agent",
 					timestamp: Date.now(),
 				},
-				"Continue if you have next steps.",
+				autoContinuePrompt,
 				{ skipPostPromptRecoveryWait: true },
 			);
 		};