docs(plan): add Fase 15 — common items management (MVP2 reopened to 7 fases)

Adds a planned-phase doc for the next ask on top of the closed MVP2 cycle:
weighted suggestion ordering + exclude items already on the active list.

- Extends item_frequency (migration 026) with `weight integer` written via
  two admin-only SECURITY DEFINER RPCs (set_item_frequency_weight +
  purge_item_frequency). The client never writes weight directly — the
  existing all-deny policies on item_frequency stay in place.
- fetchSuggestions reorders by `weight desc, use_count desc, last_used_at
  desc` and accepts an excludeNames arg to drop already-added items from
  the dropdown.
- UI lives in /collective/manage as a 3-way segmented control per row
  (Hide / Normal / Boost mapping to -50 / 0 / 50). Admin writes; member
  read-only; guest hidden.

Rebrand: MVP2 reopens from 6/6 ✅ to in-progress 6/7 ✅, with Fase 15
pending. Headers in fases 9–14 bump from "·N/6" + "fases 9 → 14" to
"·N/7" + "fases 9 → 15". README status table + index updated, CLAUDE.md
project status appends the Fase 15 pending paragraph.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

plan/fase-14-pwa-hardening.md

@@ -1,6 +1,6 @@
 ### Fase 14 — PWA hardening (auto-update + offline polish + version chip)
 
-**MVP2 · 6/6.** Última fase del ciclo MVP2 (rama `mvp2`, fases 9 → 14).
+**MVP2 · 6/7.** Última fase del ciclo MVP2 (rama `mvp2`, fases 9 → 15).
 
 **Estado: 📋 Planificada. Pulido del PWA que Fase 6 dejó funcional pero crudo. Sin migraciones de DB. Depende de Fase 9.3 (wiring de `sync_conflicts`) para el badge de conflictos.**