Harden configuration validation

docs/cli.md

@@ -47,6 +47,8 @@ Rules:
 - `--campaign` and `--campaign-file` are mutually exclusive.
 - `--session` is not used by `session init`.
 - if both positional `<session_id>` and `--session-id` are provided, values must match.
+- `--previous-session-id` is a strict expectation: the selected session file
+  must contain the same `previous_session_id`.
 - `clean --all` cannot be combined with campaign/session selectors.
 
 ## Session ID Input Rules

docs/config.md

@@ -49,7 +49,10 @@ remote state with an unsafe legacy identity must be migrated before use.
 
 ## Validation and Merge Rules
 
-- YAML decode is strict (`KnownFields(true)`): unknown fields fail load.
+- YAML decode is strict (`KnownFields(true)`) and accepts exactly one document:
+  unknown fields or trailing documents fail load.
+- 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.
 - Pipeline defaults are applied before validation.
 - Campaign and session identities must agree.
@@ -150,8 +153,8 @@ Rules:
 | `pipeline.campaigns.root` | string | No | `/usr/local/share/narratio/campaigns` |
 | `pipeline.campaigns.default_campaign_id` | string | No | empty |
 | `pipeline.secrets.env_dir` | string | No | empty |
-| `pipeline.storage.backend` | string | No | empty |
-| `pipeline.storage.s3.bucket` | string | Conditional | required for S3 session-audio and for publish upload when backend is `s3` |
+| `pipeline.storage.backend` | string | No | `local`; supported values are `local` and `s3` (case-insensitive) |
+| `pipeline.storage.s3.bucket` | string | Conditional | required when backend is `s3` and S3 session-audio or publish upload is enabled |
 | `pipeline.storage.s3.root_prefix` | string | No | `dnd` |
 | `pipeline.storage.s3.region` | string | No | empty |
 | `pipeline.storage.s3.endpoint` | string | No | empty |
@@ -316,6 +319,20 @@ For each artifact input `pipeline.scriptorium.artifacts.<name>.inputs.<input_nam
 Audio rules:
 
 - configure local mode (`audio_dir` or `audio_files`) or S3 mode (`audio_s3.prefix`), not both.
+- `audio_s3` requires `pipeline.storage.backend: s3` and a configured S3 bucket.
+
+### Storage backend selection
+
+`local` is the default and disables remote object-store operations. Configure
+`s3` explicitly before supplying `storage.s3`; a populated S3 block does not
+select a backend on its own. Unknown backend names and an S3 block paired with
+`local` are rejected during configuration validation.
+
+### Previous-session expectation
+
+`previous_session_id` is optional in a session file. When a command supplies
+`--previous-session-id`, however, the session file must contain the same value;
+an omitted or different value is rejected before the command performs work.
 
 ## Maintained Examples
 

docs/roadmap/implementation.md

@@ -745,6 +745,8 @@ effects and give the configuration package focused, non-duplicative tests.
 durations, backend combinations, strict previous expectations, environment
 isolation, defaults, unknown fields, and stable actionable errors.
 
+**Status:** Completed.
+
 ## Stage 22 — Make product configuration truthful and own remote temp files
 
 **Read first:** `audit-findings.md` lines 2521–2552 (COR-024), 3157–3197