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/src/index.ts

@@ -0,0 +1,52 @@
+/**
+ * `@deepseek-ai/dsh-web-search-perplexity`: registers a Perplexity-backed
+ * `WebSearchProvider` with `ctx.web`. A function/namespace plugin (NOT a
+ * default-export service): it registers INTO the seam's provider registry, like
+ * `@deepseek-ai/dsh-llm-deepseek` registers an adapter into `ctx.llm`.
+ *
+ * @module @deepseek-ai/dsh-web-search-perplexity
+ */
+
+import type { Context } from 'cordis'
+import z from 'schemastery'
+import type {} from '@deepseek-ai/dsh-web'
+import { PerplexitySearchProvider, PERPLEXITY_DEFAULT_BASE_URL, PERPLEXITY_DEFAULT_MODEL } from './provider.ts'
+
+export {
+  PERPLEXITY_DEFAULT_BASE_URL,
+  PERPLEXITY_DEFAULT_MODEL,
+  PERPLEXITY_PROVIDER_ID,
+  PerplexitySearchProvider,
+  mapPerplexityResponse,
+  mapPerplexityResult,
+} from './provider.ts'
+export type { PerplexitySearchProviderOptions } from './provider.ts'
+
+/** Cordis plugin name used by loader diagnostics. */
+export const name = 'web-search-perplexity'
+
+/** The web seam this provider registers into. */
+export const inject = ['web']
+
+export interface Config {
+  /** Perplexity API key. Falls back to `$PERPLEXITY_API_KEY`. Empty → unavailable. */
+  apiKey?: string
+  /** Endpoint base; `/chat/completions` is appended. Defaults to the public API. */
+  baseURL?: string
+  /** Search model name. Defaults to `sonar`. */
+  model?: string
+}
+
+export const Config: z<Config> = z.object({
+  apiKey: z.string(),
+  baseURL: z.string(),
+  model: z.string(),
+})
+
+/** Register the Perplexity search provider with `ctx.web`. */
+export function apply(ctx: Context, config: Config): void {
+  const apiKey = config.apiKey ?? process.env.PERPLEXITY_API_KEY ?? ''
+  const baseURL = config.baseURL ?? PERPLEXITY_DEFAULT_BASE_URL
+  const model = config.model ?? PERPLEXITY_DEFAULT_MODEL
+  ctx.web.registerSearchProvider(new PerplexitySearchProvider({ apiKey, baseURL, model }))
+}