docs(20): create phase plan

.planning/ROADMAP.md

@@ -759,8 +759,10 @@ Plans:
 **Goal:** Replace the per-member-row action buttons (Rotate/Add credential + Reset password) in the admin Members panel with a single edit affordance — clicking a member's name or an edit button opens a member-detail editor where an admin modifies all of that member's details in one place: display name, local-login password, and the Fastmail/CalDAV app password (calendar credential) — using clear, non-jargon labels that retire the confusing "Rotate" term. Also collapse the "Add member" section so its input fields are hidden behind a single "Add member" trigger by default, decluttering the panel. Client-side AdminPage + CredentialSheet rework over the existing `/api/admin` endpoints; no new auth/authorization boundary (seeded by the gripe that "Rotate" for the app password is not intuitive).
 **Requirements**: TBD (refine in /gsd-discuss-phase 20 — open scope: which fields count as "all" (color swatch? admin toggle? OIDC link?), whether to keep any standalone reset-password flow, and the exact edit affordance — clickable name vs. row edit button)
 **Depends on:** Phase 19
-**Plans:** 0 plans
+**Plans:** 3 plans
 
 Plans:
 
-- [ ] TBD (run /gsd-plan-phase 20 to break down)
+- [ ] 20-01-PLAN.md — Server: PATCH /api/admin/members/:id (displayName + is_admin) with last-admin demotion guard (TDD) + isAdmin in GET /members
+- [ ] 20-02-PLAN.md — PWA API client: AdminMember.isAdmin field + updateMemberProfile fetcher (last-admin sentinel)
+- [ ] 20-03-PLAN.md — PWA: unified MemberEditorSheet (edit/create, per-section saves) + decluttered tappable Members panel; retire Rotate/Reset-password buttons