bakery: add system prefix installs for BOS

Default remains ~/.local. Setting prefix=/usr/local (via
/etc/bakery/config.toml or BAKERY_PREFIX) installs bins and share
under that prefix and systemd user units under /usr/lib/systemd/user.
Writes that need root use sudo -n, then pkexec. State stays per-user.

bakery/README.md

@@ -0,0 +1,30 @@
+# bakery
+
+Package manager for the bread ecosystem. Usage lives in the
+[repo README](../README.md).
+
+## Install prefix
+
+Default root is `~/.local` (bins in `~/.local/bin`, data in
+`~/.local/share`). That is the hermes / `get.sh` path and must stay the
+default.
+
+BOS sets a system prefix so bakery-managed desktop apps live on the `@`
+root subvolume and are included in snapper/grub-btrfs snapshots:
+
+```toml
+# /etc/bakery/config.toml
+prefix = "/usr/local"
+```
+
+`BAKERY_PREFIX` overrides the config file. A non-home prefix installs:
+
+| Thing | Path |
+|-------|------|
+| bins | `$prefix/bin` |
+| share / desktop / licenses / data | `$prefix/share/...` |
+| systemd user units | `/usr/lib/systemd/user` |
+
+Per-user state (`installed.json` and pre-update backups) stays in
+`~/.local/state/bakery`. Writes that need root use `sudo -n`, then
+`pkexec`. `bakery doctor` prints the active prefix.