Fixed command substitution behavior in shell expansion:

packages/bare-os-booter/lib/shell.js
$(...)/backticks are now enabled by default unless explicitly disabled via BARE_OS_SHELL_CMDSUBST=0|false|off.
Fixed sh -c exit/status propagation:

packages/bare-os-booter/lib/shell.js
Syncs BARE_OS_EXIT_STATUS after each executed statement so later commands in the same line (like echo $?) see the immediately previous status.
packages/bare-os-coreutils/src/sh.js
In -c mode, reads final BARE_OS_EXIT_STATUS back into ctx.exitCode for consistent result propagation.
Implemented trap behavior for requested scope (EXIT, INT, TERM):

packages/bare-os-booter/lib/shell.js
exit builtin now runs EXIT trap handler before requesting booter exit.
packages/bare-os-booter/index.js
Signal delivery for shell PID now triggers trap dispatch for INT/TERM.
Hardened ulimit -f invalid diagnostics:

packages/bare-os-coreutils/src/ulimit.js
Explicit invalid-value error for malformed -f setter input, with nonzero exit.
Supported-but-unimplemented setter values still return explicit unsupported-setter error.

kernel/bin/ulimit

@@ -150,6 +150,12 @@ async function run(ctx, argv) {
   }
   if (args[0] === '-f') {
     if (args.length > 1) {
+      const raw = String(args[1] ?? '').trim()
+      if (!/^(unlimited|[0-9]+[kKmMgG]?)$/.test(raw)) {
+        ctx.console.error('ulimit: invalid file size: ' + raw)
+        ctx.exitCode = 1
+        return
+      }
       ctx.console.error('ulimit: setting file size limit is not supported in Bare OS')
       ctx.exitCode = 1
       return