ai: honour response_cache for local reviews

[shardulsdk-mpiric]

AiSettings carries response_cache, but only the daemon ever consulted
it. sashiko review builds its provider with create_provider_from_ai,
which is uncached, so turning the setting on did nothing on that path and
there was nothing in the output to say so.

create_provider_cached cannot be reused there, since it derives the cache
location from settings.database.url and a local review holds AiSettings
alone with no database beside it. This adds create_provider_cached_from_ai,
which places the cache under the XDG data directory already used for the
prompt bundle and lets SASHIKO_RESPONSE_CACHE override it. The helper that
resolves that directory is now shared rather than copied. response_cache
still defaults to false, so nothing changes unless it is switched on.

It matters most on a retry. A run that fails part way re-issues every
earlier call, and with no cache each of those is paid for again. One such
retry after a late-stage failure cost me $3.33 against a hosted endpoint,
which is what sent me looking at this.

make check-all passes, and there is a test for the path resolution
covering both the environment override and the XDG default. I am relying on
#480 having made the cache key safe across provider configurations rather
than having exercised that myself.

[kees]

I could use this! :)

I think it should use a common method, though. This is kind of duplicating code from the daemon. What about consolidating it like attached?

Also, I think the original PR suffered from creating two caches since the stdio workers under the daemon would end up creating the non-daemon cache location.
0001-ai-honour-response_cache-for-local-reviews.patch