Add explicit pipeline configuration imports

docs/config.md

@@ -55,6 +55,10 @@ remote state with an unsafe legacy identity must be migrated before use.
 
 - YAML decode is strict (`KnownFields(true)`) and accepts exactly one document:
   unknown fields or trailing documents fail load.
+- A pipeline file may explicitly import additive YAML fragments through the
+  root-only `composition.imports` list. Imported files contribute fields to one
+  logical pipeline document; they do not override fields supplied by the root
+  or another import.
 - Configured timeout and retry-delay durations must be positive. An omitted
   artifact timeout continues to inherit its configured Scriptorium timeout.
 - Session files must be concrete; unresolved `{{ ... }}` placeholders fail load.
@@ -68,6 +72,38 @@ remote state with an unsafe legacy identity must be migrated before use.
   - local (`audio_dir` or `audio_files`), or
   - S3 (`audio_s3.prefix`).
 
+### Pipeline imports
+
+Large pipeline configurations may be split into explicitly named fragments:
+
+```yaml
+composition:
+  imports:
+    - config/storage.yml
+    - config/integrations.yaml
+
+campaigns:
+  root: /usr/local/share/narratio/campaigns
+```
+
+Imports are resolved relative to the directory containing the root pipeline
+file and are loaded in declaration order. Narratio does not scan directories or
+infer fragments. Each import must be a confined regular `.yml` or `.yaml` file:
+absolute paths, traversal, symlinks, directories, duplicate files, and an
+import of the root pipeline itself are rejected. Only the root pipeline may
+contain `composition`; nested composition is rejected.
+
+Composition is additive. A map may be extended by multiple files when every
+leaf is distinct, but a scalar, list, or map/list/scalar kind cannot be claimed
+more than once, even when the repeated values are identical. Conflict errors
+name the full field path and every source that claimed it. The assembled YAML is
+then decoded against the normal strict pipeline schema and defaults are applied
+once.
+
+An imported field has the same meaning it would have in a monolithic root
+pipeline. In particular, ordinary relative pipeline paths continue to resolve
+from the root pipeline directory, not from the importing fragment's directory.
+
 ## Minimal Working Configuration
 
 `pipeline.yml`
@@ -155,6 +191,7 @@ Rules:
 
 | Field | Type | Required | Default / Rule |
 | --- | --- | --- | --- |
+| `composition.imports[]` | list of strings | No | explicit additive pipeline fragments relative to the root pipeline directory; `.yml` or `.yaml` regular files only |
 | `pipeline.workspace.root` | string | No | `/var/lib/narratio` |
 | `pipeline.workspace.cleanup_after_publish` | bool | No | `false` |
 | `pipeline.campaigns.root` | string | No | `/usr/local/share/narratio/campaigns` |