docs(tools): document fetch native tool

[konglyyy]

Summary

• document the GET-only http.fetch.get native tool in the existing Agent Tools workflow
• explain exact and wildcard URL patterns, limits, public HTTPS restrictions, and untrusted responses
• publish the updated control-plane OpenAPI contract
• clarify native-tool invocation scopes, including PDF target-chat support

Validation

• npm run validate passed with the supported bundled Node.js runtime
• documentation checks and Mintlify production build passed
• workspace platform-contract check passed

Related PRs and merge order

1. control-plane — feat(agents): add secure fetch native tool control-plane#17
2. execution-engine — feat(tools): route and redact fetch tool results execution-engine#6
3. management-console — feat(agents): configure fetch native tool management-console#19
4. docs-website (this PR) — docs(tools): document fetch native tool #12

Documentation

This is the user-facing documentation consumer and should merge after the producer and runtime/UI consumers.

Residual risk

The published OpenAPI is generated from the coordinated control-plane contract and must remain synchronized with control-plane PR 17.