Function expand_env_vars 

pub fn expand_env_vars(s: &str) -> String

P4 (design §5.2 “P4”, §1.8: “env substitution in values”): expand ${VAR} references in s against the process environment. Applied at HarnessConfig::to_config_profile to the string-valued [core] fields that plausibly vary per deployment — base_url, system_prompt, additional_dirs, extra_headers values, and extra_body string values (judgment call, §1.8’s “in values” wording names no exhaustive field list; api_key_env/api_key_cmd are deliberately EXCLUDED — the former is already an env var NAME not a value, the latter is a shell command the shell itself expands when it runs, see the call site’s comment).

An unset variable is left LITERAL (${VAR} stays in the output) rather than silently substituted with an empty string — a config author sees immediately that something didn’t resolve instead of silently getting a blank base_url/header/etc. Only the braced ${NAME} form is recognized — no bare $NAME, no shell-style :-default operators — the smallest form that satisfies the obligation without inventing a shell-expansion dialect.