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.

docs/4-implementation/turn-loop/claude-invocation.md

@@ -253,26 +253,25 @@ needs to `chown` a bind mount), once at startup:
   race), trading that for a hard dependency on the daemon's uptime
   (`Restart=always`, no stdio fallback). Extra servers stay stdio.
 - `claude-system-prompt.md` — rendered from
-  `hive-agent/prompts/system.md` by `hive-agent`'s `prompt::render`:
-  HTML-comment markers (`<!-- role:agent -->...<!-- /role:agent -->`,
-  same for `role:manager`) gate the role-specific blocks; both flavors
-  share everything else. `<!-- config:repo -->` and
-  `<!-- config:archetype -->` blocks gate the config-repo paragraphs: an
+  `hive-agent/prompts/system.md` by `hive-agent`'s `prompt::render`: a
+  [Jinja template](../../../hive-template/README.md) (`{{ name }}` for a
+  value, `{% if %}` for a block). An
+  `{% if not archetype %}` / `{% else %}` pair gates the config-repo paragraphs: an
   agent with `HIVE_ARCHETYPE` set (the meta flake sets it on an agent built
-  from an [archetype](../../2-handbook/swarm/archetypes.md)) gets the archetype block and
-  every other agent the repo block, with `{archetype}` filled in with the
-  archetype's name. Five placeholders are then
-  substituted: `{label}` (short agent name), `{qualified_label}`
+  from an [archetype](../../2-handbook/swarm/archetypes.md)) gets the archetype
+  block, with `{{ archetype }}` filled in with the archetype's name, and every
+  other agent the repo block. Five variables are
+  set: `{{ label }}` (short agent name), `{{ qualified_label }}`
   (hive-qualified `name@domain` form),
-  `{hive_identity}` (for example `` on hive `pr1ma` ``; empty when
-  `HYPERHIVE_HIVE_NAME` is unset), `{swarm_identity}` (same
-  shape for the swarm), and `{operators}`: a paragraph listing the
+  `{{ hive_identity }}` (for example `` on hive `pr1ma` ``; empty when
+  `HYPERHIVE_HIVE_NAME` is unset), `{{ swarm_identity }}` (same
+  shape for the swarm), and `{{ operators }}`: a paragraph listing the
   swarm's operators with the pronouns from each one's forge profile,
   for example `Operators: mara (she/her), sol.`. The harness reads
   them with one direct get on the one key of the `operator-profiles`
   KV bucket, which `swarm-controller` writes. The harness waits up to five seconds for
   them. When the bucket is empty or doesn't answer in time,
-  `{operators}` is empty and the prompt names no operator. Either way
+  `{{ operators }}` is empty and the prompt names no operator. Either way
   the template tells agents to say "the operator" in third person. A
   pronoun change reaches an agent at its next harness start, since
   the prompt renders once per start.
@@ -281,17 +280,11 @@ needs to `chown` a bind mount), once at startup:
   telling the agent where the docs live.
   Passed via `--system-prompt-file`.
 
-  **Marker grammar.** `<!-- KIND:X -->` opens a block; any
-  `<!-- /KIND:X -->` closes the current block of that kind. The renderer
-  filters the two kinds one after the other. The renderer always uses
-  role `agent`, so it elides blocks with other role tags, and config `repo`
-  or `archetype` as above. Nesting **isn't**
-  supported — a stray opener with no closer runs until end of file.
-  The renderer tolerates whitespace inside markers (`<!--role:foo-->` parses the
-  same as `<!-- role:foo -->`). Content outside any marker is always
-  included. Today's `system.md` carries no role markers (single agent
-  role), and the role grammar stays wired for a future manager / multi-role
-  prompt; it carries one pair of config blocks.
+  **Template rules.** A variable the template names but the renderer doesn't
+  set is an error. The harness then writes the template unrendered and logs the
+  reason, so a typo shows up in the prompt instead of as an empty string. A
+  block tag on a line of its own leaves no blank line behind. A lone `{` or `}`
+  is plain text.
 
   **`hive_identity` / `swarm_identity` shape.** Each carries a
   leading space + backticked name (` on hive \`pr1ma\``,