fix(home): make stateVersion overridable

Changed home.stateVersion assignment to use lib.mkDefault instead of a plain assignment.
This fixes downstream flakes that need to migrate existing NixOS installations without bumping their state version.

Verified: V1 (flake check --no-build, home activation package build).
Pending: nothing.

agent/BACKLOG.md

@@ -283,15 +283,7 @@ actually float once seen (regex tolerance for the `.…-wrapped` form).
 *Agents: append here with a one-paragraph pitch (what/why/cost). Do not
 implement. Bernardo moves accepted items into a tier.*
 
-- **`home.stateVersion` should be `mkDefault`** (surfaced writing
-  docs/MIGRATION.md, 2026-07-08). `modules/home/default.nix:51` sets
-  `home.stateVersion = "26.05"` as a *plain* assignment, so a downstream
-  home.nix that needs a different value (anyone migrating an existing NixOS
-  install — their original install's stateVersion) can only override it with
-  `lib.mkForce`, which reads as a hack. Cheap fix: make it
-  `lib.mkDefault "26.05"` so a plain downstream assignment wins cleanly,
-  matching the mkDefault discipline the rest of the distro follows. (The
-  system side is already fine — the downstream owns system.nix outright.)
+
 
 - **Battery charge-limit — make the toggle instant** `[blocked:hw]`
   (follow-up to the preset toggle shipped 2026-07-07, iteration #55).