compass add capture should search for related stories/captures first

compass add capture currently scaffolds a brand-new capture file unconditionally, with no check for whether an existing capture, task, or story already covers the same topic. Before writing the new file, it should search the corpus (title/description/filetags, similar to compass search) for close matches and, if any are found, show them to the user and ask whether to update/link the existing document instead of creating a duplicate.

This session hit the exact case the feature would catch: while working the is_sweepable task, a fresh RLS-policy gap was found and about to be captured as a new file, but codegen_rls_policy_facet.org already existed covering precisely this recurring class of gap (NATS handler registrar, tenant-provisioning copy block, RLS policies – all "codegen writes the entity but something still has to be hand-wired afterwards"). The right move was to add this occurrence as further evidence to the existing capture, not create a new one – but that required manually searching first; nothing prompted it.

Duplicate captures fragment the same underlying idea across multiple files, diluting its evidence trail (each recurrence should strengthen one capture's case for prioritisation, not scatter across N near- identical ones) and making the backlog harder to triage – a reviewer has to notice the duplication themselves, the same problem this feature would solve for the person filing the capture in the first place.

• doc/agile/product_backlog/inbox/codegen_rls_policy_facet.org – the capture this session correctly found and updated by hand instead of duplicating, illustrating the workflow this feature would automate.