feat(mail-templates): markdown inputs in a multi-line field (1.0.3)

description and callout_text declare format: markdown, which Riposte
0.322+ edits in a text area that keeps line breaks; older versions
ignore it and \n still works. README documents format: multiline.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BJu1jvaug4qY1kMgz8wx41

README.md

@@ -141,6 +141,18 @@ issue_type_name:
 Riposte refuses an `options_from` command that is not `risk: read`: opening a
 field must never change anything in the customer's tool.
 
+### Multi-line inputs
+
+`format: multiline` (or `format: markdown`, which says what the text is) makes
+the playbook editor show a resizable text area instead of a one-line field
+(Riposte 0.322+). Use it for a mail body, a comment, a markdown description:
+a one-line field turns the line breaks of a pasted text into spaces. Older
+Riposte versions ignore `format` and keep the one-line field.
+
+```yaml
+description: { type: string, format: markdown, description: "Body in markdown" }
+```
+
 ### `risk:` — the agent approval gate
 
 Every command declares a tier. It is what Riposte's agent approval gate reads: