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).

tsconfig.base.json

@@ -34,6 +34,8 @@
       "@cordisjs/plugin-timer": ["./vendor/timer/src"],
       "@cordisjs/plugin-hmr": ["./vendor/hmr/src"],
       "@cordisjs/plugin-logger-console": ["./vendor/logger-console/src"],
+      "@deepseek-ai/dsh-tool-web/search": ["./packages/web/tool-web/src/search.ts"],
+      "@deepseek-ai/dsh-tool-web/fetch": ["./packages/web/tool-web/src/fetch.ts"],
       // One wildcard maps every @deepseek-ai/dsh-<name> to its source. Package
       // dir names are unique across groups, so first-on-disk-wins resolution is
       // unambiguous; adding a package under an existing group needs no edit
@@ -45,6 +47,7 @@
         "./packages/bash/*/src",
         "./packages/compact/*/src",
         "./packages/subagent/*/src",
+        "./packages/web/*/src",
         "./packages/session-persistence/*/src",
         "./packages/ui/*/src",
         "./packages/util/*/src",