Caching and ETag/If-Modified-Since support

[pokymono]

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.

[Ryuga-17]

@pokymono I am working in this issue.

[vrd-cse]

i can do this work. assign me the job

[Ryuga-17]

i have raised a pr, please review it. #35