Show when each extension was last updated

MCP, skill and tool rows in Settings say "Updated 5m ago" beside how
they're reached and who gets them. updated_at now means a change to an
extension's settings (an edit, or any Access save): runs, connections and
MCP syncs keep their own last_checked_at / last_synced_at and no longer
touch it.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/harness/tools/bot/extensions/merge-custom-tool-changes.test.ts

@@ -12,6 +12,7 @@ const weather: CustomToolRecord = {
     status: "ok",
     last_error: "",
     last_checked_at: null,
+    updated_at: null,
     kind: "http",
     config: {
         method: "GET",

src/harness/tools/skills/open-bot-skills.test.ts

@@ -12,6 +12,7 @@ function skill(overrides: Partial<SkillRecord> & { slug: string }): SkillRecord
         body: `${overrides.slug} body`,
         all_bots: false,
         enabled: true,
+        updated_at: null,
         ...overrides,
     };
 }