Add remote session loading

docs/config.md

@@ -6,7 +6,7 @@ Narratio loads three YAML files:
 
 - `pipeline.yml`: pipeline-level runtime configuration.
 - `campaign.yml`: stable campaign identity and campaign-level input defaults.
-- `session.yml`: per-session metadata and input selection.
+- `session.yml`: per-session metadata and input selection, loaded locally or from the configured S3 backend.
 
 These commands load and validate all three files before running:
 
@@ -20,6 +20,7 @@ Behavior:
 
 - strict YAML decode is enabled (`KnownFields(true)`): unknown fields fail.
 - session templates render before session YAML decode.
+- remote `session.yml` uses the same strict decode and template behavior as local `session.yml`.
 - defaults are applied for optional pipeline fields.
 - campaign-level stable input paths fill missing session input paths.
 - session-level stable input paths override campaign-level input paths.
@@ -49,11 +50,14 @@ Campaign config lookup for `run`, `plan`, `resume`, `run-stage`, and `restore`:
 Session config lookup for `run`, `plan`, `resume`, `run-stage`, and `restore`:
 
 - if `--session <path>` is provided, that path is used.
-- if omitted, Narratio searches in order:
+- if `--session` is omitted, Narratio searches locally in order:
   1. `./session.yml`
   2. `/usr/local/etc/narratio/session.yml`
   3. `/etc/narratio/session.yml`
-- first existing file wins.
+- first existing local file wins.
+- if no local session file is found, `--session-id <value>` is present, storage is configured, and campaign identity is resolved, Narratio loads remote `session.yml` from:
+  - `{root_prefix}/campaigns/{campaign}/sessions/{session_id}/session.yml`
+- local discovery always runs before remote fallback.
 
 Template behavior: