Function link_history_if_isolated 

pub(crate) fn link_history_if_isolated(
    paths: &DataPaths,
    dir: &Path,
    provider: &ProviderConfig,
    entity_id: &str,
    log_ctx: &str,
)

Best-effort: if provider has a linkable native history directory (ProviderConfig::history_native_subdir, None for providers with no fixed on-disk history to redirect) and credential isolation is currently on, ensure dir/<subdir> is linked to the always-global history location for entity_id (a bundle id or account id — same namespace, see [DataPaths::identity_dir]). No-op when isolation is off (dir is already inside the global tree, so dir/<subdir> already IS the global location) or the provider has no linkable subdir. Never fails the caller — logs and continues, same philosophy as every other soft-fail in this module.

Three call sites need this identically: compute_and_ensure_bundle_dir and compute_and_ensure_account_dir below (both auth.start-time), and inject::inject_identity_env_with_broker’s OAuth branch (the ordinary spawn-time path, which reads a previously stored OAuthConfigDir and — before this shared helper existed — never re-verified the link at all; reagentx P1 on PR #2605: an account provisioned before this whole feature shipped, or since its last auth.start, never got linked until this fix). One function so they can’t drift on the condition again.