fix(ai): keep render_chart when chart skill is explicitly selected

In explicit chart auto mode, the filter at agent/index.ts removed
render_chart whenever the user message lacked chart keywords. This
caused failures when a user manually selected the built-in chart skill
with a message like "analyze message trends" — the skill prompt
instructed the AI to produce charts but the tool was unavailable.

Add an isExplicitChartSkill guard so the filter is skipped when
skillCtx.skillDef.id === CHART_CAPABILITY_SKILL_ID.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/desktop/main/ai/agent/index.ts

@@ -10,6 +10,7 @@ import { getHistoryForAgent, setPendingDebugContext } from '../chats'
 import { aiLogger, isDebugMode } from '../logger'
 import { t as i18nT } from '../../i18n'
 import {
+  CHART_CAPABILITY_SKILL_ID,
   DEFAULT_MAX_TOOL_ROUNDS,
   buildPlanGuidance,
   createAnalysisPlanner,
@@ -130,7 +131,12 @@ export class Agent {
     const allowedTools = this.assistantConfig?.allowedBuiltinTools
     const toolContext = { ...this.context, locale: this.locale }
     let piTools = getAllTools(toolContext, allowedTools)
-    if (this.config.chartAutoMode === 'explicit' && !shouldUseChartCapabilityForMessage(userMessage)) {
+    const isExplicitChartSkill = this.skillCtx?.skillDef?.id === CHART_CAPABILITY_SKILL_ID
+    if (
+      this.config.chartAutoMode === 'explicit' &&
+      !isExplicitChartSkill &&
+      !shouldUseChartCapabilityForMessage(userMessage)
+    ) {
       piTools = piTools.filter((tool) => tool.name !== 'render_chart')
     }