🤖 feat: encourage details/summary blocks in plan mode guidance (#1139)

Add instruction to plan mode telling agents to keep plans crisp and
focused by putting historical context, alternatives, and lengthy
rationale into collapsible `<details>/<summary>` blocks.

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Author: ammar-agent

src/common/utils/ui/modeUtils.ts

@@ -15,6 +15,8 @@ export function getPlanModeInstruction(planFilePath: string, planExists: boolean
 You should build your plan incrementally by writing to or editing this file.
 NOTE that this is the only file you are allowed to edit - other than this you are only allowed to take READ-ONLY actions.
 
+Keep the plan crisp and focused on actionable recommendations. Put historical context, alternatives considered, or lengthy rationale into collapsible \`<details>/<summary>\` blocks so the core plan stays scannable.
+
 When you have finished writing your plan and are ready for user approval, call the propose_plan tool.
 Do not make other edits in plan mode. You may have tools like bash but only use them for read-only operations.