feat: conditional fetching with ETag/If-Modified-Since support

[chaliy]

What

Support conditional HTTP fetching so agents can efficiently poll pages without re-downloading unchanged content.

Approach

• Add etag and last_modified to FetchResponse (already have last_modified)
• Add if_none_match (ETag) and if_modified_since fields to FetchRequest
• When server returns 304 Not Modified, return response with status_code: 304 and no content
• Return etag header value in response for future conditional requests

Why

Agents frequently re-fetch the same URLs in loops. Conditional fetching avoids re-downloading and re-processing unchanged pages.

Acceptance criteria

• New request fields: if_none_match, if_modified_since
• New response field: etag
• 304 responses handled correctly (no content, no conversion)
• Tests with wiremock covering conditional request/response flow