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-9-polish-dark-mode.md

@@ -1,6 +1,6 @@
 ### Fase 9 — Polish + Dark Mode
 
-**MVP2 · 1/6.** Primera fase del ciclo MVP2 (rama `mvp2`, fases 9 → 14).
+**MVP2 · 1/7.** Primera fase del ciclo MVP2 (rama `mvp2`, fases 9 → 15).
 
 **Estado: ✅ Completada (2026-05-18). Ver `docs/history/fase-9-polish-dark-mode.md` para detalles + paleta definitiva. 41 tests nuevos: 8 user_theme pgTAP, 12 sync_conflicts pgTAP, 4 sync-conflicts integración, 3 SC unit, 8 theme unit, 3 theme E2E, 3 reading-width E2E. WebKit specs (2 nuevos) gated en `RUN_WEBKIT=1`.**