keep the dynamism of make in pure shell

Two mechanisms, matching what the make engine actually did:

Lazy defaults. A stack setting is a function myos_default_<VAR>, called only
when the variable has no value, and called again at every reference. That is
exactly a recursive ?=: an explicit value wins, and the default follows a
DOMAIN that a .env changes later. The prefix is what makes it safe; the first
version used a bare function named after the variable, and the test suite
caught it running /usr/bin/host for a stack group called host.

Templates. myos env-update fills a .env from the .env.dist files, expanding
${VAR} against the current values and running $(command), forward references
included.

Also fixed: the project .env now wins over /etc/conf.d/myos, which is what the
documentation claimed and the code did not.

share/make/shim.mk lets a project keep make as a front end: every myos command
becomes a target that shells out to bin/myos, and the project keeps its own
targets and its stack .mk files. It sits outside make/ because the legacy
engine globs every .mk in there.

CHANGELOG.md

@@ -12,6 +12,14 @@
 - agent skill in `skills/myos/`, contributor notes in `AGENTS.md`
 - stacks carry their settings in `<name>.env` and `<name>.sh` hooks, so the
   catalogue no longer needs make to be installed
+- lazy defaults (`myos_default_<VAR>` functions) give the recursive `?=` of
+  make in pure shell: an explicit value wins, and the default is recomputed
+  at each reference
+- `myos env-update` generates a `.env` from the `.env.dist` templates,
+  expanding `${VAR}` and `$(command)`, including forward references
+- the project `.env` now wins over `/etc/conf.d/myos`, as documented;
+  `MYOS_CONF_PRIORITY=system` restores the previous order
+- `share/make/shim.mk`: make as an optional front end over the same shell code
 - `--color always|never|auto`, and no colour when the output is piped
 - verified under the /bin/sh of Alpine (busybox) and Debian (dash)
 - the make engine still works and is still covered by the golden tests