Add secrets directory credential resolver

docs/config.md

@@ -72,6 +72,7 @@ Sidecar generation writes `report.html` for `report.md`. It does not mutate the
 
 Top level:
 
+- `secrets.directory`: optional credential secrets directory.
 - `pipelines`: required non-empty list.
 
 Pipeline:
@@ -168,7 +169,18 @@ Defaults are applied after YAML decoding and before validation:
 
 ## Secrets
 
-Do not put literal secrets in config files. S3 credentials may name environment variables:
+Do not put literal secrets in config files. `secrets.directory` lets deployments provide credential values as files:
+
+```yaml
+secrets:
+  directory: /run/secrets/distributor
+```
+
+Each regular file in the directory becomes an internal credential environment value named by the filename. Valid filenames must match `[A-Za-z_][A-Za-z0-9_]*`. Directories are ignored, and symlinks to regular files are followed. Exactly one trailing LF or CRLF is trimmed from each file; other whitespace is preserved.
+
+The resolver checks the real process environment first, then the secrets directory. If both define the same variable with different values, `run` prints a warning with the variable name and uses the real environment value. Secret values are not printed. The process environment is not modified, so SDK default credential chains see only real environment variables.
+
+S3 credentials may name environment variables:
 
 - `credentials.access_key_id_env`
 - `credentials.secret_access_key_env`