templates: the agent system prompt is a Jinja template, rendered through hive-template

system.md carries {{ label }}-style variables and an {% if not archetype %} block in place of the config:repo / config:archetype marker pairs, so prompt.rs drops its line filter, marker parsers and string replaces. The rendered prompt is byte for byte the old one for a custom agent, an archetype agent and an empty archetype name, with and without hive, swarm, docs and operators. hive-template now drops a block tag's own line.

hive-template/README.md

@@ -7,6 +7,9 @@ block is `{% if cond %}...{% endif %}`, and a lone `{` or `}` stays plain text.
 - **`render(template, &context)`**: the text, or a message when the template
   does not parse or names a variable the context lacks. An unknown variable is
   an error, never an empty string.
+- A block tag alone on its line leaves no line behind (`trim_blocks` and
+  `lstrip_blocks` are on), so a template ending in a block tag and a newline
+  loses that newline.
 - **`variables(template)`**: the variables the template reads, so a caller can
   refuse an unknown field when the template is saved instead of when it renders.
 

hive-template/src/lib.rs

@@ -10,12 +10,15 @@ use std::collections::BTreeSet;
 use minijinja::{Environment, UndefinedBehavior};
 use serde::Serialize;
 
-/// An environment that refuses a name the context does not hold, and keeps the
-/// template's trailing newline.
+/// An environment that refuses a name the context does not hold, keeps the
+/// template's trailing newline, and drops a block tag's own line (`{% if %}` on a
+/// line by itself leaves no blank line behind).
 fn environment() -> Environment<'static> {
     let mut env = Environment::new();
     env.set_undefined_behavior(UndefinedBehavior::Strict);
     env.set_keep_trailing_newline(true);
+    env.set_trim_blocks(true);
+    env.set_lstrip_blocks(true);
     env
 }
 
@@ -56,7 +59,16 @@ mod tests {
             "{{ repo }}{% if role == \"agent\" %} (agent){% endif %}\n",
             &ctx,
         );
-        assert_eq!(out.as_deref(), Ok("o/r (agent)\n"));
+        assert_eq!(out.as_deref(), Ok("o/r (agent)"));
+    }
+
+    #[test]
+    fn a_block_tag_on_its_own_line_leaves_no_line_behind() {
+        let out = render(
+            "a\n{% if on %}\nb\n{% else %}\nc\n{% endif %}\nd\n",
+            &json!({ "on": false }),
+        );
+        assert_eq!(out.as_deref(), Ok("a\nc\nd\n"));
     }
 
     #[test]