docs(agent): PROPOSED #134 — unstable.<pkg> in the downstream

Bernardo wants a newer LM Studio and rejected the only mechanism that works
today (a hand-pinned fetchTarball rev+sha256 in home.nix — verified working,
but a second unlocked pin and exactly the Nix expertise the distro exists to
spare its users). He is taking stable 0.4.15-2 for now.

Records why there is no seam (mkFlake takes src/username/hardwareProfile/
system; homeConfigurations uses mkFlake's own pkgs, so a system.nix overlay
cannot reach a home package), the sketch he asked for (unstable.lmstudio,
maybe stable. as an explicit synonym), and the shape that keeps the one-input
promise: carry nixpkgs-unstable upstream and expose it via overlays.default,
so downstream stays single-input and locked.

Costs left open for triage, not decided: two-channel lock churn against the
full-checklist rule, closure duplication (measured: the two lmstudio build
closures share 690 of ~3.7-4k paths), the weakening of "tested together
upstream", whether an unreferenced input should lock at all, and whether the
seam covers system.nix too. Cheaper fallback noted (mkFlake `overlays ? []`).

PROPOSED per BACKLOG rules — awaiting triage, not implemented.

agent/BACKLOG.md

@@ -249,6 +249,38 @@ high-ROI, etc.) live in the journal + ROADMAP — not here.*
 
 ### Product / day-2
 
+### 134. `unstable.<pkg>` in the downstream — a newer app without a second flake
+
+Bernardo 2026-07-15, wanting a newer LM Studio (pinned 0.4.15-2 vs unstable
+0.4.19-2) and disliking the only path that works today. There is **no seam**:
+`mkFlake` takes `src`/`username`/`hardwareProfile`/`system` only, and
+`homeConfigurations` is built from mkFlake's own `pkgs` (`inherit pkgs`), so a
+`nixpkgs.overlays` in system.nix cannot reach a home package. The user is left
+hand-pinning a `fetchTarball` rev+sha256 in home.nix (verified working) — a
+second pin with no lock, which is exactly the Nix expertise the distro exists
+to spare them. Sketch: `home.packages = [ unstable.lmstudio ];`, and maybe
+`stable.` as an explicit synonym for today's bare `pkgs`.
+
+The shape that keeps the promises: Nomarchy carries the `nixpkgs-unstable`
+input **itself** and exposes it through `overlays.default` as an `unstable`
+attrset, so downstream stays **one input**, stays locked, and `nomarchy-pull`
+still moves it. Cost/questions to settle before building:
+- Lock churn: every lock bump now moves two channels, and the full-checklist
+  rule (VERIFICATION §5) applies to both.
+- Duplication: measured — pinned vs unstable lmstudio build closures share
+  only 690 of ~3.7–4k paths (separate stdenv bootstrap). Opt-in per package,
+  but a user who reaches for it pays a second glibc/stack in the closure.
+- "Tested together upstream" weakens by construction: nothing validates
+  pinned-Nomarchy + unstable-app. Needs an explicit, documented "you own this
+  combination" line, not silence.
+- Does `unstable` belong in the eval by default, or only when referenced?
+  (An unused input still locks + fetches.)
+- Scope: home packages only, or system.nix too?
+
+Alternative if that is too much surface: give `mkFlake` an `overlays ? []`
+param (one-line seam, user still owns their own input/pin — cheaper for us,
+still expert-only for them). Related: #133.
+
 ### 114. Greeter ignores per-device keyboard layouts
 
 Found by Bernardo 2026-07-14: logging out while docked lands on tuigreet,