Skip file content reads in search when query doesn't reference content

JsonLogic search was calling cachedRead() on every note in the vault,
even for queries that only inspect frontmatter. On large vaults this
caused ~3s response times for simple frontmatter lookups.

Now searchJsonLogic checks whether the serialized query contains the
string "content" before reading file bodies. Queries that only touch
frontmatter, tags, path, links, etc. skip cachedRead entirely and
receive an empty string for the content field, which is correct since
they never inspect it.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>