feat(agent-core): slim WebSearch to query-only; fetch page content via FetchURL (#1260)

* feat(agent-core): slim WebSearch to query-only; fetch page content via FetchURL

The coding search backend no longer honors `limit`/`enable_page_crawling` and
always returns full page content, which was token-heavy and frequently
truncated. Realign the web tools around a search+fetch split:

- WebSearch request sends only `text_query`; the tool exposes only `query`.
- Search results drop inline page content and add the source site; full page
  content is now fetched on demand via FetchURL.
- Add a citation reminder to both WebSearch and FetchURL results (in the
  FetchURL front note so it survives body truncation).
- Update tool descriptions and reference docs accordingly.

* fix(agent-core): satisfy no-base-to-string lint and drop redundant | undefined

- Assert the exact serialized WebSearch request body instead of String()-coercing
  a BodyInit, fixing the type-aware no-base-to-string lint error.
- Drop redundant `| undefined` from WebSearchResult optional fields per AGENTS.md.
- Add changeset.

* chore(changeset): bump agent-core to minor for WebSearch input-contract change

Removing the `limit`/`include_content` tool inputs tightens a closed
(`additionalProperties: false`) schema, so previously-valid args are now
rejected — an incompatible change for a released package. Bump minor rather
than patch.

.changeset/web-search-query-only.md

@@ -0,0 +1,6 @@
+---
+"@moonshot-ai/agent-core": minor
+"kimi-code-docs": patch
+---
+
+WebSearch now sends only the query and returns lightweight result summaries (title, source site, date, URL, snippet) instead of inlined page content; fetch a result's full page content on demand with FetchURL. Both tools now include a citation reminder in their results.