feat(sessions): surface live session startup warnings in the web UI

Show a pinned banner at the top of the session view with resource and
runtime diagnostics (skills, prompts, themes, extension load errors) plus
the Anthropic subscription-auth billing notice, recomputed live from the
current runtime so they stay accurate across browser reloads.

Warnings carry an optional dismiss capability; the Anthropic notice is
dismissable and durably suppressed through pi's own anthropicExtraUsage
warning setting. Also fixes the testing-guide skill frontmatter so it
loads.

src/server/sessions/sessionRoutes.ts

@@ -189,6 +189,15 @@ export function registerSessionRoutes(app: FastifyInstance, sessions: SessionRou
     }
   });
 
+  app.post<{ Params: { sessionId: string }; Body: { cwd?: unknown; dismissId?: unknown } | undefined }>(`${prefix}/sessions/:sessionId/warnings/dismiss`, async (request, reply) => {
+    try {
+      const body = optionalRecord(request.body);
+      return await sessions.dismissWarning(sessionLookupFromBody(request.params.sessionId, body), requireString(body, "dismissId"));
+    } catch (error) {
+      return reply.code(mutationErrorStatus(error)).send({ error: errorMessage(error) });
+    }
+  });
+
   app.post<{ Params: { sessionId: string }; Body: AttachmentsRequestBody | undefined }>(`${prefix}/sessions/:sessionId/attachments`, async (request, reply) => {
     try {
       const body = optionalRecord(request.body);