Harden API key file loading

docs/config.md

@@ -94,7 +94,13 @@ inputs:
 
 - Do not place raw secrets in YAML.
 - Use env var names in config (for example `pipeline.audita.llm_api_key_env`).
-- Optionally load env files from `pipeline.secrets.env_dir`.
+- Optionally load credential files from `pipeline.secrets.env_dir`. Each valid
+  environment-variable filename supplies one value; trailing CR/LF is removed.
+- An existing process environment value takes precedence over a credential file.
+- Credential directories and files must not be symlinks and must be regular,
+  bounded files (at most 8 KiB per value). On POSIX, provision the directory
+  with no group/other access (normally `0700`) and files with no group/other
+  access (normally `0600`).
 - Commands that need storage/auth load filesystem secrets before constructing adapters.
 
 ## Publish Configuration Summary