docs(fase-18): history doc + CHANGELOG [Unreleased] entry

Records the four asks shipped in Fase 18 — required list title, auto-
numbered prefix from a catalog match, admin-curated catalog + per-
collective default — alongside the migration / pgTAP / store / UI /
e2e changes that landed across the cycle. Notes the four design
decisions worth remembering (DB stays #N-agnostic, no unique
constraint, auto-suffix only on catalog hits, bare-hash refuses to
autonumber) plus the explicit out-of-scope items.

CHANGELOG.md gains a single `[new]` line under `[Unreleased]` per the
orchestrator's spec.

Final test counts after Fase 18:
  pgTAP        191  (was 178; +13)
  Integration  185  (was 179; +6)   +3 skipped
  Unit          75  (was 56;  +19)
  Playwright   104  (was 100; +4)
  Gated          1  (unchanged)
  Total        555  +3 skipped       (was 514 +3)

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

CHANGELOG.md

@@ -9,6 +9,8 @@ labels for the area, not log entries.
 
 ## [Unreleased]
 
+- [new] Required list title + auto-numbered prefix + admin-curated title catalog (Fase 18).
+
 ## [v0.0.0-beta] — 2026-05-19 — MVP + MVP2 (Fases 0–16)
 
 Single rollup for everything shipped before tagging started. Covers the