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/share/man/man.json

@@ -1,6 +1,6 @@
 {
   "schemaVersion": 1,
-  "generatedAt": "2026-04-26T19:04:18.469Z",
+  "generatedAt": "2026-04-26T19:19:08.715Z",
   "pages": [
     {
       "name": "agent",
@@ -3211,8 +3211,13 @@
       "synopsis": [
         "sh [OPTION]... [OPERAND]..."
       ],
-      "description": "Runs a shell script file line-by-line via ctx.execLine (same language as the interactive booter shell). This is not the interactive REPL; see man bare-os-shell for session builtins, pipelines, and jobs.",
-      "options": [],
+      "description": "Runs shell commands via ctx.execLine (same language as the interactive booter shell). Supports command-string mode (`-c`) and script-file mode (`sh SCRIPT`). This is not the interactive REPL; see man bare-os-shell for session builtins, pipelines, and jobs.",
+      "options": [
+        {
+          "flag": "-c COMMAND [NAME [ARG ...]]",
+          "meaning": "Execute command string and set positional parameters for that command context."
+        }
+      ],
       "keywords": [
         "sh",
         "bare-os",