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)**