sketch: add support for 'external' message types

- adds a new CodingAgentMessageType for loop.AgentMessage
- adds an new /external handler to loophttp.go
- modifies Agent to pass the .TextContent of ExternalMessage into the convo
  as though it came from the user.
- adds sketch-external-message web component, with a template for
  github workflow run events specifically.
- adds demos for sketch-external-message
diff --git a/webui/src/web-components/sketch-external-message.ts b/webui/src/web-components/sketch-external-message.ts
new file mode 100644
index 0000000..1df27d8
--- /dev/null
+++ b/webui/src/web-components/sketch-external-message.ts
@@ -0,0 +1,128 @@
+import { html, TemplateResult } from "lit";
+import { customElement, property, state } from "lit/decorators.js";
+import { ExternalMessage } from "../types";
+import { SketchTailwindElement } from "./sketch-tailwind-element";
+import type { WorkflowRunEvent } from "@octokit/webhooks-types";
+
+@customElement("sketch-external-message")
+export class SketchExternalMessage extends SketchTailwindElement {
+  @property() message: ExternalMessage | null = null;
+  @property() open: boolean;
+  @property() summaryContent: TemplateResult | string = "";
+  @property() detailsContent: TemplateResult | string = "";
+  @state() detailsVisible: boolean = false;
+
+  _toggleDetails(e: Event) {
+    e.stopPropagation();
+    this.detailsVisible = !this.detailsVisible;
+  }
+
+  constructor() {
+    super();
+  }
+
+  render() {
+    if (this.message?.message_type === "github_workflow_run") {
+      const run = this.message.body as WorkflowRunEvent;
+      this.summaryContent = html`<svg
+          class="inline-block w-4 h-4"
+          viewBox="0 0 16 16"
+          width="16"
+          height="16"
+        >
+          <path
+            fill="currentColor"
+            d="M8 0C3.58 0 0 3.58 0 8c0 3.54 2.29 6.53 5.47 7.59.4.07.55-.17.55-.38 0-.19-.01-.82-.01-1.49-2.01.37-2.53-.49-2.69-.94-.09-.23-.48-.94-.82-1.13-.28-.15-.68-.52-.01-.53.63-.01 1.08.58 1.23.82.72 1.21 1.87.87 2.33.66.07-.52.28-.87.51-1.07-1.78-.2-3.64-.89-3.64-3.95 0-.87.31-1.59.82-2.15-.08-.2-.36-1.02.08-2.12 0 0 .67-.21 2.2.82.64-.18 1.32-.27 2-.27.68 0 1.36.09 2 .27 1.53-1.04 2.2-.82 2.2-.82.44 1.1.16 1.92.08 2.12.51.56.82 1.27.82 2.15 0 3.07-1.87 3.75-3.65 3.95.29.25.54.73.54 1.48 0 1.07-.01 1.93-.01 2.2 0 .21.15.46.55.38A8.013 8.013 0 0016 8c0-4.42-3.58-8-8-8z"
+          ></path>
+        </svg>
+        <span
+          class="inline-flex items-center px-1.5 py-1
+        ${run.workflow_run.conclusion === "success"
+            ? "bg-green-600/10 text-green-600"
+            : run.workflow_run.conclusion === "failure"
+              ? "bg-red-600/10 text-red-600"
+              : "bg-gray-600/10 text-gray-600"}
+          rounded-md text-xs flex-shrink-0 transition-colors"
+          >${run.workflow.name} -
+          ${run.workflow_run.conclusion || run.workflow_run.status}</span
+        >
+        on ${run.workflow_run.head_branch} at ${run.workflow_run.head_commit}`;
+
+      this.detailsContent = html`
+        <div class="flex items-center gap-2">
+          <span class="text-gray-700 dark:text-gray-300">Workflow ID:</span>
+          <span class="font-mono font-medium text-gray-800 dark:text-gray-200">${run.workflow_run.id}</span>
+        </div>
+        <div class="flex items-center gap-2">
+          bg-gray-600/10 text-gray-600
+        {{end}}
+          rounded-md text-xs flex-shrink-0 transition-colors'><a target="_blank"
+            rel="noopener noreferrer"href="${run.workflow_run.html_url}">${run.workflow.name} - ${run.workflow_run.conclusion}</a>
+            on ${run.workflow_run.head_branch} at ${run.workflow_run.head_commit}</span>
+            </span>`;
+
+      this.detailsContent = html`
+        <div class="flex items-center gap-2">
+          <span class="text-gray-700 dark:text-gray-300">Workflow Run ID:</span>
+          <span class="font-mono font-medium text-gray-800 dark:text-gray-200"
+            >${run.workflow_run.id}</span
+          >
+        </div>
+        <div class="flex items-center gap-2">
+          <span class="text-gray-700 dark:text-gray-300">Status:</span>
+          <span class="font-mono font-medium text-gray-800 dark:text-gray-200"
+            >${run.workflow_run.status}</span
+          >
+        </div>
+        <div class="flex items-center gap-2">
+          <span class="text-gray-700 dark:text-gray-300">Conclusion:</span>
+          <span class="font-mono font-medium text-gray-800 dark:text-gray-200"
+            >${run.workflow_run.conclusion}</span
+          >
+        </div>
+        <div class="flex items-center gap-2">
+          <span class="text-gray-700 dark:text-gray-300">Run URL:</span>
+          <a
+            class="text-blue-600 dark:text-blue-400 hover:underline"
+            href="${run.workflow_run.html_url}"
+            target="_blank"
+            rel="noopener noreferrer"
+          >
+            ${run.workflow_run.html_url}
+          </a>
+        </div>
+      `;
+    }
+
+    return html`
+      <div class="block max-w-full w-full box-border overflow-hidden">
+        <div class="flex flex-col w-full">
+          <div
+            class="flex w-full box-border py-1.5 px-2 pl-3 items-center gap-2 cursor-pointer rounded relative overflow-hidden flex-wrap hover:bg-black/[0.02] dark:hover:bg-white/[0.05]"
+            @click=${this._toggleDetails}
+          >
+            <span
+              class="whitespace-nowrap flex-grow flex-shrink text-gray-700 dark:text-gray-300 font-mono text-xs px-1 min-w-[50px] max-w-[calc(100%-150px)] inline-block"
+              >${this.summaryContent}</span
+            >
+          </div>
+          <div
+            class="${this.detailsVisible
+              ? "block"
+              : "hidden"} p-2 bg-black/[0.02] dark:bg-white/[0.05] mt-px border-t border-black/[0.05] dark:border-white/[0.1] font-mono text-xs text-gray-800 dark:text-gray-200 rounded-b max-w-full w-full box-border overflow-hidden"
+          >
+            ${this.detailsContent
+              ? html`<div class="mb-2">${this.detailsContent}</div>`
+              : ""}
+          </div>
+        </div>
+      </div>
+    `;
+  }
+}
+
+declare global {
+  interface HTMLElementTagNameMap {
+    "sketch-notification-message": SketchExternalMessage;
+  }
+}