sh: add POSIX-like -c support and complete shell subset gaps

Implement sh -c COMMAND [NAME [ARG...]] in /bin/sh, add shell function
declarations/invocation support, expand export semantics (NAME, NAME=value, -p),
and make until/loop-control behavior available by default. Add focused shell
tests and update sh man-page option docs.

packages/bare-os-seeder/kernel/etc/bare-os/posix_utilities.json

@@ -1,7 +1,7 @@
 {
   "schema": 2,
   "profileId": "bare-os-posix-like",
-  "generatedAt": "2026-04-26T19:04:18.264Z",
+  "generatedAt": "2026-04-26T19:19:08.534Z",
   "note": "Sparse POSIX Issue 7 coverage hints for /bin utilities. Omitted command names are not yet profiled here.",
   "commandIndex": [
     {