Add PostgREST query builder to SupabaseClient

Implements TableQueryBuilder with select/eq/order/limit/insert/update/execute
over sync httpx, closing the .table() gap between SupabaseClient and the
SupabaseClientLike protocol that ItemRepository and AgentRunRepository depend on.
Adds 26 focused tests covering param composition, repo behavior, and mocked
HTTP round-trips.

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