feat(flake): #134 — unstable.<pkg> in the downstream, one input, no hand pins

Nomarchy now carries a nixpkgs-unstable input and exposes it through
overlays.default as an `unstable` attrset: in the template's
`home.packages = with pkgs; [ … ]` a user writes `unstable.lmstudio`
and is done. Downstream stays one locked input; nomarchy-pull moves it.
lib.nix needed zero changes — mkFlake's pkgs already applies the
overlay. The import is plain and non-recursive, so configs that never
reference unstable. pay nothing beyond the input fetch; allowUnfree
mirrors the base set so the motivating lmstudio resolves.

Home packages are the supported surface (documented convention). Costs
stated in the README, not hidden: you own the pinned+unstable
combination; a second toolchain lands in your closure (~690 of ~4k
paths shared, measured on lmstudio); it moves when the lock moves.
Lock bumps now move two channels — VERIFICATION §5 and CONVENTIONS
updated accordingly. flake.lock gained exactly one node
(nixos-unstable @ 753cc8a); nothing else moved.

Verification: V1 — nix flake check --no-build green; new eval-only
checks.unstable-seam green (26.05pre vs 26.11pre lib.version, no
builds); template HM activation built via mkFlake with unstable.hello
enabled and its closure carries the unstable store path (pg2zfr… vs
pinned nm7p8w…). Implementation by a Sonnet subagent; design and
review on Fable.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -519,6 +519,17 @@ reload                          # exec zsh (reload the shell)
   line to slim the machine, uncomment an extra (a browser, email, full TeX
   Live), or add your own. No `nomarchy.apps.*` toggles — a package list is
   already its own opt-out, so the distro doesn't impose these or wrap them.
+- **A newer individual app (unstable channel, #134):** Nomarchy carries a
+  second, newer nixpkgs channel and exposes it as `unstable` through its
+  overlay, so any package can be taken from it by prefixing `unstable.` in
+  `home.packages` — e.g. `unstable.lmstudio` — with no second flake input of
+  your own to manage. Three costs, stated plainly: (a) **you own this
+  combination** — nothing upstream tests a pinned Nomarchy plus an unstable
+  app together; (b) an unstable package brings its own second toolchain into
+  your closure (measured: `unstable.lmstudio` shares only ~690 of ~4k store
+  paths with the pinned set); (c) it moves whenever the flake lock moves
+  (`nomarchy-pull`), not on its own schedule. Home packages only — that's
+  the supported surface, not `system.nix` or the module set.
 
 ## Roadmap & known issues