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/tsdown.config.ts

@@ -0,0 +1,18 @@
+import { defineConfig } from 'tsdown'
+
+/**
+ * tool-web exposes one package root plus one entry per tool plugin, so each tool
+ * can be loaded or replaced independently as a subpath plugin
+ * (`@deepseek-ai/dsh-tool-web/search`, `/fetch`). The root tsdown config only
+ * auto-discovers `src/index.ts`, so the subpath entries are declared here.
+ */
+export default defineConfig({
+  entry: ['src/index.ts', 'src/search.ts', 'src/fetch.ts'],
+  outDir: 'lib',
+  format: ['esm'],
+  platform: 'node',
+  target: 'es2024',
+  fixedExtension: false,
+  dts: false,
+  clean: false,
+})