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/piSessionService.testSupport.ts

@@ -87,6 +87,7 @@ export function fakeRuntime(sessionId = "session-1", patch: Partial<TestSession>
     isBashRunning: false,
     pendingMessageCount: 0,
     sessionManager: fakeSessionManager(),
+    settingsManager: { getWarnings: () => ({}), setWarnings: () => undefined },
     modelRegistry: ModelRegistry.create(AuthStorage.inMemory()),
     scopedModels: [],
     extensionRunner: { getRegisteredCommands: () => [] },