Add web capability seam: ctx.web, search/fetch providers, web tools

Introduce web access as a first-class capability seam so the model-facing
web tools stay stable while backends change. dsh-web owns ctx.web as a
provider registry with registration-order-independent selection and the
WebError taxonomy; dsh-web-search-exa, dsh-web-search-perplexity, and
dsh-web-fetch-local register capabilities into it; dsh-tool-web is the sole
owner of the model-facing web_search/web_fetch schemas, prompt sections, and
HTML-to-markdown presentation. Search and fetch are deliberately one seam.

Providers ship as namespace plugins that register into ctx.web (like an
LlmAdapter into ctx.llm), not key-owning services, since multiple search
providers cannot each own the key. Tool registration follows product
enablement, not backend availability, so load order/credentials never enter
the model contract; the seam resolves the provider at execution time and
surfaces a structured WebError otherwise.

Moves the RFC to implemented/ amended to match what shipped. Example/app
configs are intentionally not wired yet (RFC migration step 6).

packages/web/web-search-perplexity/tests/perplexity.e2e.ts

@@ -0,0 +1,23 @@
+import { describe, expect, it } from 'vitest'
+import { PerplexitySearchProvider, PERPLEXITY_DEFAULT_BASE_URL, PERPLEXITY_DEFAULT_MODEL } from '@deepseek-ai/dsh-web-search-perplexity'
+
+/**
+ * Real-API smoke for the Perplexity search provider. Self-skips without
+ * `$PERPLEXITY_API_KEY`, per the with-key e2e policy in AGENTS.md § Secrets.
+ */
+const apiKey = process.env.PERPLEXITY_API_KEY
+const maybe = apiKey !== undefined && apiKey.length > 0 ? describe : describe.skip
+
+maybe('PerplexitySearchProvider real API', () => {
+  it('returns a generated answer and sources for a live query', async () => {
+    const provider = new PerplexitySearchProvider({
+      apiKey: apiKey!,
+      baseURL: process.env.PERPLEXITY_BASE_URL ?? PERPLEXITY_DEFAULT_BASE_URL,
+      model: process.env.PERPLEXITY_MODEL ?? PERPLEXITY_DEFAULT_MODEL,
+    })
+    const result = await provider.search({ query: 'What is the DeepSeek coding agent?', maxResults: 5 })
+    expect(result.providerId).toBe('perplexity')
+    expect(result.content ?? '').not.toBe('')
+    for (const source of result.sources) expect(source.url).toMatch(/^https?:\/\//)
+  }, 30_000)
+})