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/tool-web/src/index.ts

@@ -0,0 +1,59 @@
+/**
+ * The model-facing web tool suite (`web_search`, `web_fetch`) over the `ctx.web`
+ * seam. This root plugin registers the tools the product has ENABLED, composing
+ * the per-tool registration helpers; each tool is also exposed as a subpath
+ * plugin (`@deepseek-ai/dsh-tool-web/search`, `/fetch`) for focused deployments.
+ *
+ * The package owns model-facing concerns only — tool names, JSON schemas,
+ * argument validation, prompt sections, result-cap constants, result formatting,
+ * HTML→markdown presentation. All web access goes through `ctx.web`; this
+ * package never imports a concrete provider package.
+ *
+ * Tool registration follows product/app ENABLEMENT, not backend availability: a
+ * tool stays visible even when its selected provider is missing/misconfigured,
+ * and execution fails with a structured `WebError` (resolved by the seam at call
+ * time). That keeps the model schema stable without making plugin load order,
+ * credential state, or HMR timing part of the model-facing contract.
+ *
+ * @module @deepseek-ai/dsh-tool-web
+ */
+
+import type { Context } from 'cordis'
+import z from 'schemastery'
+import type {} from '@deepseek-ai/dsh-web'
+import { applyWebSearchTool } from './search.ts'
+import { applyWebFetchTool } from './fetch.ts'
+
+export { WEB_SEARCH_MAX_RESULTS, applyWebSearchTool, formatSearchOutput, parseSearchArgs, presentSearchCall } from './search.ts'
+export { applyWebFetchTool, formatFetchOutput, parseFetchArgs, presentFetchCall, renderBody } from './fetch.ts'
+export { htmlToMarkdown } from './html.ts'
+
+/** Cordis plugin name used by loader diagnostics. */
+export const name = 'tool-web'
+
+/** Services required by the web tool suite. */
+export const inject = ['tools', 'web', 'systemPrompt']
+
+export interface Config {
+  /** Register `web_search`. Defaults to true. */
+  search?: boolean
+  /** Register `web_fetch`. Defaults to true. */
+  fetch?: boolean
+}
+
+export const Config: z<Config> = z.object({
+  search: z.boolean().default(true),
+  fetch: z.boolean().default(true),
+})
+
+/**
+ * Register the enabled web tools. `search`/`fetch` default to true; a product
+ * that wants only one disables the other in config. The tools' disposers are
+ * fiber-scoped (the effect-based registries clean up on dispose), so no manual
+ * teardown is needed.
+ */
+export function apply(ctx: Context, config: Config): void {
+  if (config.search !== false) applyWebSearchTool(ctx)
+  if (config.fetch !== false) applyWebFetchTool(ctx)
+}
+