Add session discovery and template support

docs/roadmap/narratio-s3-archive-implementation-plan.md

@@ -95,6 +95,11 @@ Implemented in repository:
   - campaign/session/run local work and spool path helpers
   - manifest run/path identity fields
 - examples and tests for the above foundations
+- session template operator UX:
+  - default session config discovery (`./session.yml`, `/usr/local/etc/narratio/session.yml`, `/etc/narratio/session.yml`)
+  - `--session-id` template injection for `session_id`
+  - session template rendering before strict YAML decode
+  - unresolved template placeholders and `session_id` mismatches fail clearly
 - remote storage backend layer:
   - object-store abstraction with `List`, `Download`, `Upload`, and `Exists`
   - fake storage backend for deterministic, no-network testing