feat(toolkit): align fetchkit with toolkit library contract

[chaliy]

What

Align fetchkit with the Everruns toolkit library contract while keeping the existing typed fetch API working.

Why

Everruns expects toolkit libraries to expose a predictable metadata, schema, and execution surface. fetchkit had overlapping pieces but not the standard contract shape, which forced bespoke integration.

How

• add the contract surface: locale-aware ToolBuilder factories, Tool metadata methods, ToolExecution, ToolOutput, ToolError, and a generic tower::Service
• keep legacy typed APIs as compatibility wrappers over the new contract
• update MCP to delegate schema/name/validation through the library contract while preserving markdown-by-default behavior
• extend tests and README coverage for the new contract surface

Risk

• Medium
• Tool metadata now uses the stable contract name web_fetch, which changes MCP tool listing and external integrations that relied on the old fetchkit tool name
• Help and description output changed shape to satisfy the contract, though feature gating remains covered by tests

Checklist

• Unit tests are passed
• Smoke tests are passed
• Documentation is updated
• Specs are up to date and not in conflict
• Security review completed