Function resolve_secret 

pub fn resolve_secret(secret_ref: &SecretRef) -> Result<String, ResolverError>

Resolve a SecretRef to the plaintext credential string. Each backend has a distinct path:

• Env: read env_var from the srv process’s own environment. Caller is expected to have set this in their shell or a .env-style loader before launching AgentMux.
• PlaintextDev: return the literal stored string. Debug builds only — guarded behind cfg(debug_assertions). In release builds, the same call returns ResolverError::PlaintextDevDisabledInRelease so a forgotten dev-secret never leaks into a packaged binary. Reagent P1 on PR #751 caught the missing guard. Phase 3’s encrypted vault is the production path.
• SecretsManager: deferred. Returns ResolverError::SecretsManagerUnsupported.