Pin bread-theme to ecosystem v0.7.1; document BOS wiring and pacman-only exception

Point workspace bread-theme at git.breadway.dev tag v0.7.1 (gtk feature stays
on breadgreet). README no longer claims BOS ships tuigreet or that Super+L
is unimplemented. CONTRIBUTING records the single-trunk, pacman-only model;
there is still no bakery.toml.

CONTRIBUTING.md

@@ -0,0 +1,22 @@
+# Contributing
+
+`breadlock` / `breadgreet` — session locker and greetd greeter for Hyprland.
+
+Single-trunk, same as the rest of the ecosystem: one long-lived branch
+(`main`), short-lived `feature/<name>` / `fix/<name>` branches, merge back.
+No `dev` or `beta` branch.
+
+This repo is a deliberate **pacman-only** exception. There is no
+`bakery.toml` — breadlock needs a root-owned `/etc/pam.d/breadlock` PAM
+service, which bakery cannot install. Don't add one. Releases are `v*`
+tags that fire `.forgejo/workflows/package.yml` into the `[breadway]`
+pacman repo. There are no bakery tracks.
+
+See `CLAUDE.md` (local hygiene notes) for remotes and CI details.
+
+## Local development
+
+```sh
+cargo build --release --bin breadlock --bin breadgreet
+cargo test --workspace
+```