feat(core): expose fetch to code mode scripts - #49235

Scripts run by the execute tool can now call fetch. Built on #49196 (extensions are host functions) and #49242 (before/after hooks): one host function, plain globalThis.fetch, closures for the response, no new runtime types.

const res = await fetch("https://api.example.com/users", { method: "POST", body: JSON.stringify({ name }) })
if (!res.ok) throw new Error(`HTTP ${res.status}`)
return await res.json()

Three commits:

 packages/core/src/codemode/
+├── web.ts        Extension.make({ name: "web", globals: { fetch } }) + display(args) → { method, url }
 └── tool.ts
+  CodeMode.make({ tools, extensions: [CodeModeWeb.extension], hooks })
+  "extension.before": fetch rows get input: { method, url } so the TUI's existing summary shows them
-  "Do not use `fetch`; all external access goes through `tools`."
+  "`fetch` is available for HTTP requests. … all other external access goes through `tools`."

fetch(url, init?) takes method, headers, and a string / Uint8Array / URLSearchParams body, and resolves to { url, status, statusText, ok, redirected, headers: { get, has, entries }, text(), json(), bytes() }. 30 s timeout. Not a tool call; not in the catalog.

In the TUI, with no UI changes, a fetch appears under execute like a nested tool call:

✓ execute
  › fetch [method=GET, url=https://example.com]

Verified against a local server: rows appear in start order as running, settle to completed or error, and the final toolCalls metadata matches.

Not in this PR: permission prompting (webfetch's Permission.assert), body size caps, origin policy.

Checks: bun run check 35/35; packages/core test/tool-execute.test.ts + test/codemode 27 pass.