docs(agent): NEXT #76 hibernation default + zram

Promote hibernation+zram product intent to the queue head; [big] with
design/module/installer/V3 slices. FUTURE stub points at #76.

agent/BACKLOG.md

@@ -24,11 +24,34 @@ in [`docs/ROADMAP.md`](../docs/ROADMAP.md); map in
 
 ## NOW
 
-*(empty — NEXT is empty; see LATER / FUTURE / PROPOSED)*
+*(empty — NEXT's top item is the queue head)*
 
 ## NEXT
 
-*(empty — no actionable queue head; #20 deferred to FUTURE)*
+### 76. Hibernation default + zram `[big]`
+**Product intent (Bernardo 2026-07-10):** hibernation on by default;
+zram on alongside for live memory pressure. Disk swap ≥ RAM for the
+hibernate image; zram high-priority for day-to-day swap (not sufficient
+alone for resume).
+
+**Scope (split before/while implementing — do not land as one mega-PR):**
+
+1. **Design** — installer disk layout (swapfile size = RAM? partition?),
+   `resume`/`resume_offset`, LUKS + systemd-initrd resume, priority
+   (zram > disk), interaction with existing `@swap` / `swapSize` disko
+   path, what to do when swap was installed as `"0"`.
+2. **Module** — NixOS: `zramSwap.enable`, disk swap + hibernation
+   (`boot.resumeDevice` / swapfile resume), menu or documented power
+   path for Hibernate (already in Power menu if swap works).
+3. **Installer** — new installs create hibernation-capable swap by
+   default; document migration for existing machines (add swapfile +
+   rebuild).
+4. **Verify** — V0/V1: eval + unit/config assertions; V2: VM resume if
+   feasible; **V3**: real laptop hibernate → power off → resume
+   (HARDWARE-QUEUE). Encrypted root is the hard case.
+
+**Out of scope:** TLP changes; formatter. Prefer boring NixOS knobs over
+custom scripts.
 
 ## LATER
 
@@ -66,6 +89,19 @@ the `vm-checks` job in `.gitea/workflows/check.yml` (`runs-on: nix-kvm`,
 `nix flake check` + toplevel/HM builds). Do not enable the job until
 that label is online (Gitea queues forever otherwise).
 
+### Formatter — adopt later `[human]`
+**Intent:** add a Nix formatter (likely `nixfmt-rfc-style`) in a dedicated
+pass: reformat the tree once, document in CONVENTIONS, optional CI
+check. **Not** the queue head — no drive-by reformats until that pass.
+
+### Hibernation default + optional zram
+**Intent:** hibernation on by default (disk swap + resume). **zram is
+compatible** if disk swap remains for the hibernation image; zram only
+helps *live* memory pressure (compressed RAM swap), not resume-after-
+poweroff. Prefer: high-priority zram for day-to-day + lower-priority
+disk swap ≥ RAM for hibernate. Design before NEXT (installer layout,
+encrypted root, `resume_offset`).
+
 ## PROPOSED (agent suggestions — await human triage)
 
 *Agents: append here with a one-paragraph pitch (what/why/cost). Do not