Generic fetch_resource tool with configurable backend URL

[pavelanni]

Summary

Replace the current fetch_document tool with a generic fetch_resource tool that accepts a configurable backend URL. This allows fetching from any HTTP service, not just the document service.

Motivation

Currently fetch_document is hardcoded to a specific document service endpoint. A generic tool would make DocsClaw useful for any backend — APIs, search services, knowledge bases, etc.

Acceptance criteria

• New fetch_resource tool with configurable base URL via agent-config.yaml
• Backward-compatible with existing fetch_document configurations
• URL validation and SSRF protection preserved
• Documentation updated

From roadmap: docs/slides/docsclaw-intro.html, slide 6