Session configuration templates are now proceeded by narratio session init; all other commands require concrete configuration

docs/config.md

@@ -19,11 +19,11 @@ These commands load and validate all three files before running:
 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`.
+- ordinary local and remote `session.yml` files must be concrete YAML; template placeholders are rejected.
 - 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.
+- campaign config may point `session init` to a session template.
 - validation enforces required fields, value formats, and cross-field constraints.
 
 ## 2. Config file discovery
@@ -66,18 +66,29 @@ Session config lookup:
 
 ## 3. Session templating
 
-Template behavior for local and remote `session.yml`:
+Template behavior for local and remote `session.yml` loaded by downstream commands:
 
-- supported placeholders:
-  - `{{session_id}}`
+- downstream commands do not render templates.
+- local and remote `session.yml` must be concrete.
+- any `{{ ... }}` placeholder in loaded `session.yml` fails with guidance to run `narratio session init`.
+- if concrete `session_id` mismatches `--session-id`, load fails.
+- if concrete `previous_session_id` mismatches `--previous-session-id`, load fails.
+
+Template behavior for `narratio session init`:
+
+- `campaign.yml` may set `session_template_file`.
+- relative template paths resolve relative to `campaign.yml`.
+- supported init template variables:
   - `{{ session_id }}`
-  - `{{previous_session_id}}`
   - `{{ previous_session_id }}`
-- `--session-id <value>` supplies the placeholder value.
-- `--previous-session-id <value>` supplies the previous-session placeholder value.
-- unresolved placeholders fail load.
-- if rendered `session_id` mismatches `--session-id`, load fails.
-- if rendered `previous_session_id` mismatches `--previous-session-id`, load fails.
+  - `{{ date }}`
+  - `{{ title }}`
+  - `{{ audio_s3_prefix }}`
+  - `{{ audio_dir }}`
+- each template variable must be supplied by the matching `session init` flag.
+- template-related flags such as `--date`, `--title`, `--audio-s3-prefix`, `--audio-dir`, and `--previous-session-id` fail if the configured template does not use them.
+- rendered output is strict-decoded and validated before it is written locally or remotely.
+- if `session_template_file` is omitted, `session init` generates the minimal concrete session YAML directly.
 
 ## 4. Minimal config set
 
@@ -98,6 +109,7 @@ Why this is sufficient:
 
 ```yaml
 campaign: sample-campaign
+session_template_file: ./session.template.yml
 inputs:
   speakers_file: ./speakers.yml
   autocorrect_file: ./autocorrect.yml
@@ -112,14 +124,14 @@ Why this is sufficient:
 ### `session.yml`
 
 ```yaml
-session_id: "{{ session_id }}"
+session_id: 2026-05-03
 inputs:
   audio_dir: ./audio
 ```
 
 Why this is sufficient:
 
-- `session_id` is required and can be rendered from `--session-id`.
+- `session_id` is required.
 - `campaign` can be omitted because it is supplied by `campaign.yml`.
 - stable input paths can be omitted because `campaign.yml` supplies defaults.
 - local `audio_dir` resolves relative to `session.yml`.
@@ -133,8 +145,8 @@ narratio run --config /path/to/pipeline.yml --campaign ./campaign.yml --session
 Previous-session-enabled variant:
 
 ```yaml
-session_id: "{{ session_id }}"
-previous_session_id: "{{ previous_session_id }}"
+session_id: 2026-05-03
+previous_session_id: 2026-04-26
 inputs:
   audio_dir: ./audio
 ```
@@ -214,8 +226,8 @@ inputs:
 ### Local `session.yml`
 
 ```yaml
-session_id: "{{ session_id }}"
-previous_session_id: "{{ previous_session_id }}"
+session_id: 2026-05-03
+previous_session_id: 2026-04-26
 date: 2026-05-03
 title: The Black Cabin
 inputs:
@@ -416,11 +428,12 @@ Restore-related implications:
 | Path | Type | Required | Default |
 | --- | --- | --- | --- |
 | `campaign.campaign` | string | Yes | none |
+| `campaign.session_template_file` | string | No | none |
 | `campaign.inputs.speakers_file` | string | Yes | none |
 | `campaign.inputs.autocorrect_file` | string | Yes | none |
 | `campaign.inputs.glossary_file` | string | Yes | none |
 
-Campaign input paths may be absolute or relative. Relative paths resolve from the directory containing `campaign.yml`.
+Campaign input paths and `campaign.session_template_file` may be absolute or relative. Relative paths resolve from the directory containing `campaign.yml`.
 
 ## 8. Full session reference