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/sessionService.ts

@@ -37,6 +37,7 @@ export interface SessionRouteService {
   status(ref: SessionRouteLookup): Promise<ClientSessionStatus>;
   streamSnapshot(ref: SessionRouteLookup): Promise<SessionStreamSnapshot>;
   clearQueue(ref: SessionRouteLookup): Promise<ClientSessionStatus>;
+  dismissWarning(ref: SessionRouteLookup, dismissId: string): Promise<ClientSessionStatus>;
   availableModels(ref: SessionRouteLookup): Promise<ClientSessionModel[]>;
   setModel(ref: SessionRouteLookup, provider: string, modelId: string): Promise<ClientSessionStatus>;
   cycleModel(ref: SessionRouteLookup, direction: "forward" | "backward"): Promise<ClientSessionStatus>;