docs(migration): #76 slice 1 — enable-hibernation runbook

Add a self-contained "Enabling hibernation on an existing machine (no
reinstall)" section to docs/MIGRATION.md, per the settled #76 call to
document migration rather than ship a tool. Covers: @swap subvolume
creation (subvolid=5), btrfs mkswapfile (NOCOW), reading the resume
offset via map-swapfile, and wiring fileSystems."/swap" + swapDevices +
resumeDevice + resume_offset into system.nix. Flags the /swap mount as
required on the hand-edit path (a fresh install inherits it from
disko-generated hardware-config). Notes the zram-priority reservation,
a no-LUKS variant, and the swap=0 opt-out.

Verification: V0. All read-only commands run live against this machine's
LUKS(cryptroot)+btrfs(@)+/swap/swapfile layout; nix flake check
--no-build green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

agent/BACKLOG.md

@@ -72,7 +72,9 @@ interplay handled in `modules/nixos/default.nix` / `modules/home/idle.nix`.
 
 **Remaining slices:**
 
-1. **Docs** — `docs/MIGRATION.md` hibernation-enable runbook (V0).
+1. ~~**Docs** — `docs/MIGRATION.md` hibernation-enable runbook (V0).~~ ✓
+   shipped 2026-07-10 (§ "Enabling hibernation on an existing machine";
+   commands verified live against the dev machine's LUKS+@swap layout).
 2. **Menu** — Hibernate row: notify-on-failure when no swap configured
    (behavioral, V1/V2). Keep the row unconditional.
 3. **Verify (required)**

agent/JOURNAL.md

@@ -17,6 +17,24 @@ Template:
 
 ---
 
+## 2026-07-10 — #76 slice 1: MIGRATION.md hibernation runbook
+- **Task:** BACKLOG #76 remaining slice 1 — docs runbook for enabling
+  hibernation on an existing machine (per the settled "docs not tool" call).
+- **Did:** Added § "Enabling hibernation on an existing machine (no reinstall)"
+  to `docs/MIGRATION.md`: create `@swap` subvol (subvolid=5) + `mkswapfile`
+  (NOCOW), read `map-swapfile -r` offset, wire `fileSystems."/swap"` +
+  `swapDevices` + `resumeDevice` + `resume_offset` into system.nix. Flagged
+  the mount as required on the hand-edit path (installer gets it from
+  disko-gen hardware-config). Notes zram-priority reservation, no-LUKS
+  variant, `swap=0` opt-out. Worked example = this dev machine.
+- **Verified:** **V0** — read-only commands run live against this machine's
+  real LUKS(`cryptroot`)+btrfs(`@`)+`/swap/swapfile` layout (RAM 31G, fsuuid,
+  `findmnt SOURCE`, `mkswapfile` flags, zram@100 vs swapfile@-1 confirms the
+  design intent). `nix flake check --no-build` green.
+- **Pending:** slice 2 — Hibernate row notify-on-failure (V1/V2); then the
+  V2 hibernate VM test + V3 laptop.
+- **Next suggestion:** slice 2 (menu notify-on-failure).
+
 ## 2026-07-10 — #76 hibernation design calls (no commit yet → this bookkeeping)
 - **Task:** BACKLOG #76 design slice — Bernardo made the open calls.
 - **Did:** Recon showed the hibernation half is **already built for new