Caching and ETag/If-Modified-Since support

## Why
Avoid refetching the same URLs; speed up repeated runs by supporting HTTP caching headers and on-disk caching.

## Tasks
- Implement an optional lightweight on-disk cache keyed by URL plus relevant headers (ETag, Last-Modified, etc).
- Integrate ETag and If-Modified-Since support into HTTP fetches, respecting cache headers from origin servers.
- Allow cache bypass via configuration/CLI flag (e.g., --no-cache).
- Add docs covering caching, cache bypass, and cache location.

## Acceptance Criteria
- Cache improves speed for repeated runs on the same URLs.
- Cache bypass is configurable from CLI/config.