Fix build recipe execution and flatten reliability

- Set HOME in bctx.EnvVars when USER switches so ~ expands correctly in
  subsequent RUN/WORKDIR steps instead of resolving to /root
- Run /bin/sync inside the guest before FlattenRootfs destroys the VM,
  preventing pip-installed files from being captured as 0-byte due to
  unflushed page cache
- Wrap healthcheck command with su <user> so it runs with the template's
  default user context (correct HOME, correct UID)
- Export Shellescape from the recipe package for use in build service
- Add code-runner-beta recipe (Jupyter server with ipykernel --sys-prefix)
  and replace old python-interpreter-v0-beta

internal/recipe/context.go

@@ -115,8 +115,11 @@ func expandEnv(s string, vars map[string]string) string {
 	})
 }
 
-// shellescape wraps s in single quotes, escaping any embedded single quotes.
+// Shellescape wraps s in single quotes, escaping any embedded single quotes.
 // This is POSIX-safe for paths, env values, and shell commands.
-func shellescape(s string) string {
+func Shellescape(s string) string {
 	return "'" + strings.ReplaceAll(s, "'", `'\''`) + "'"
 }
+
+// shellescape is the package-internal alias for Shellescape.
+func shellescape(s string) string { return Shellescape(s) }