Brief merge/collapse — consolidate N briefs into one (v3 of #31)

Implementation complete on branch flower/brief-merge; not merged. Commits: df3e4ef Add brief merge core; 1b27e8a Expose brief merge provenance in recall; 46b2903 Add briefs merge UI. All include `Brief: #33`. Behavior: added briefs.merged_into_brief_id + Brief::mergedInto/mergedFrom; BriefRelationService::merge(survivorId, sourceIds[], actor) re-points source links, participants, dependencies, and children onto the survivor, copies project scopes to the survivor, cancels sources with merged_into pointers, and records merged events on survivor and sources. Guards self-merge, terminal/already-merged briefs, source active dispatch, parent-cycle merges, and dependency cycles during re-pointing. Added brief_merge MCP tool. recall_brief now surfaces merged_into/merged_from provenance. /briefs has per-row merge into action plus merged-away pointer and survivor merged count. Verification: focused service/MCP/recall/UI/Foundation/query tests passed (41 tests, 363 assertions); Pint passed; full `~/bin/php artisan test` passed (474 tests, 473 passed, 1 skipped, 3415 assertions).

Started implementation on branch flower/brief-merge from master (master includes #31 relations + #32 affinity merge). Grounded via recall_brief #33 and prior BriefRelationService/relations recall. Building additive merge/collapse only: relation-service merge, MCP tool, /briefs UI action, recall provenance; no behavior change for unmerged briefs.

## Goal (v3 of Brief #31) Editorial op to collapse/merge multiple briefs into one (pre-existing or new) — consolidating related/duplicate briefs during batch refinement. ## Scope - brief_merge(source_ids[], target_id|new, actor_ref): fold sources' specs/notes/links/participants/ dependencies into the target (append-only trace preserved; dependencies re-pointed); mark sources merged/cancelled with a pointer to the target. - UI: "merge selected briefs" action on /briefs. ## Depends on Brief #31 (relationships infra). CHAIN onto #31. ## Provenance Brief #31 refinement (2026-07-01): merge/collapse deferred to v3; capture as a brief to chain onto #31.