feat(x): inline charts in chat via a charts skill

The chat's markdown fence override now renders ```chart blocks
(the same ChartBlockSchema notes use) as interactive recharts inline —
the mermaid pattern. A new ChartRenderer handles line/bar/pie with
multi-series support, light/dark theming, and a quiet placeholder
while the fence body is still streaming.

ChartBlockSchema.y widens to string | string[] for multi-series
charts (notes chart block updated to map series too), with a
chartSeries() helper in shared.

A new 'charts' skill teaches the model the fence format, form
selection (line/bar/pie, no mixed scales), and data honesty rules;
loaded on demand like any skill.

apps/x/packages/core/src/runtime/assembly/skills/charts/skill.ts

@@ -0,0 +1,74 @@
+export const skill = String.raw`
+# Charts
+
+Load this skill when the user asks for a chart, graph, plot, trend, comparison, or "visualize" — or when data you've gathered (prices over time, counts per category, proportions) would land better as a picture than a table.
+
+## How it works
+
+Emit a fenced code block with language \`chart\` anywhere in your reply. The app renders it as an interactive chart inline (tooltips, legend, light/dark theming are handled for you — never pick colors yourself). Everything outside the fence is normal markdown.
+
+\`\`\`\`
+\`\`\`chart
+{ ...JSON config... }
+\`\`\`
+\`\`\`\`
+
+## Config schema
+
+- **\`chart\`** (required): \`"line"\` | \`"bar"\` | \`"pie"\`
+- **\`data\`** (required): array of flat objects — the rows to plot. Put REAL values you gathered this turn here; never invent numbers.
+- **\`x\`** (required): key of the label/category field in each row
+- **\`y\`** (required): key of the value field — a string for one series, or an array of keys for several series on one chart
+- **\`title\`** (optional): short heading shown above the chart
+
+## Picking the form
+
+- **line** — change over time (prices, counts by date). X is the time field.
+- **bar** — compare magnitudes across categories (issues per label, revenue per region).
+- **pie** — proportions of a whole; only with ≤ 6 slices, otherwise use a bar.
+- Two measures with very different scales (e.g. a $600 stock vs a $5 one): do NOT mix them on one chart — either normalize to % change and say so in the title, or emit two chart blocks.
+
+## Examples
+
+Multi-series line (comparison over time, normalized):
+
+\`\`\`chart
+{
+  "chart": "line",
+  "title": "5-Day % Change",
+  "x": "date",
+  "y": ["AAPL", "NVDA", "SPY"],
+  "data": [
+    { "date": "Jul 15", "AAPL": 0, "NVDA": 0, "SPY": 0 },
+    { "date": "Jul 16", "AAPL": -0.4, "NVDA": 1.2, "SPY": 0.1 },
+    { "date": "Jul 17", "AAPL": -1.1, "NVDA": 0.8, "SPY": -0.3 }
+  ]
+}
+\`\`\`
+
+Single-series bar:
+
+\`\`\`chart
+{
+  "chart": "bar",
+  "title": "Open issues by area",
+  "x": "area",
+  "y": "count",
+  "data": [
+    { "area": "sync", "count": 14 },
+    { "area": "editor", "count": 9 },
+    { "area": "billing", "count": 3 }
+  ]
+}
+\`\`\`
+
+## Rules
+
+- Data must come from what you actually fetched or computed this turn — never fabricate points to make a chart possible. Too little real data? Say so instead of charting.
+- Keep it readable: ≤ ~30 rows per chart, ≤ 6 series. Round values sensibly.
+- Numbers must be JSON numbers, not strings ("3.1", "5%" won't plot).
+- Follow the chart with one sentence of takeaway — what the reader should see in it.
+- One chart per distinct question; don't emit several variants of the same data.
+`;
+
+export default skill;

apps/x/packages/core/src/runtime/assembly/skills/index.ts

@@ -29,6 +29,7 @@ import backgroundTaskSkill from "./background-task/skill.js";
 import notifyUserSkill from "./notify-user/skill.js";
 import appsSkill from "./apps/skill.js";
 import slackSkill from "./slack/skill.js";
+import chartsSkill from "./charts/skill.js";
 
 const CURRENT_DIR = path.dirname(fileURLToPath(import.meta.url));
 const CATALOG_PREFIX = "src/runtime/assembly/skills";
@@ -77,6 +78,12 @@ const definitions: SkillDefinition[] = [
     summary: "Prepare for meetings by gathering context about attendees from the knowledge base.",
     content: meetingPrepSkill,
   },
+  {
+    id: "charts",
+    title: "Charts",
+    summary: "Render interactive charts (line, bar, pie) inline in the chat reply. Use when the user asks for a chart/graph/visualization or when gathered data is clearer as a picture.",
+    content: chartsSkill,
+  },
   {
     id: "organize-files",
     title: "Organize Files",