Add session discovery and template support

architecture.md

@@ -115,6 +115,25 @@ CLI pipeline config path resolution:
   - `/usr/local/etc/narratio/pipeline.yml`
   - `/etc/narratio/pipeline.yml`
 
+CLI session config path resolution:
+
+- when `--session <path>` is provided, that path is used
+- when `--session` is omitted, Narratio searches defaults in order:
+  - `./session.yml`
+  - `/usr/local/etc/narratio/session.yml`
+  - `/etc/narratio/session.yml`
+
+Session template rendering:
+
+- session templates are rendered before strict YAML decode
+- `--session-id <value>` provides the `session_id` template variable
+- supported placeholders:
+  - `{{session_id}}`
+  - `{{ session_id }}`
+- unresolved placeholders fail clearly
+- strict `KnownFields(true)` YAML validation still applies after rendering
+- if rendered `session.session_id` conflicts with `--session-id`, load fails clearly
+
 Optional pipeline secrets directory:
 
 - `pipeline.secrets.env_dir` enables loading environment variables from local files before command execution