hive-ag3nt + docs: extract prompt-rendering prose (#716 batch 3)

docs/turn-loop.md

@@ -187,6 +187,26 @@ socket at `/run/hive/` once at startup:
   `services.hyperhive.c0re.operatorPronouns`, default `she/her`).
   Passed via `--system-prompt-file`.
 
+  **Marker grammar.** `<!-- role:X -->` opens a block; matching
+  `<!-- /role:X -->` closes it. Nesting is NOT supported — a stray
+  opener overrides until its closing tag (or end of file). A
+  mismatched closer (`<!-- /role:manager -->` inside a `role:agent`
+  block) is elided from the output but does NOT pop the active
+  role: suppression stays conservative so a typo can't dump
+  wrong-flavor content. Whitespace inside markers is tolerated
+  (`<!--role:foo-->` parses the same as `<!-- role:foo -->`).
+  Content outside any marker is always shared.
+
+  **`hive_identity` / `swarm_identity` shape.** Each carries a
+  leading space + backticked name (` on hive \`pr1ma\``,
+  ` in swarm \`constellat1on\``) when the corresponding env var
+  is set, otherwise empty string. The independence lets the
+  template drop one or both into the opener prose without
+  breaking single-hive deployments that never set the option;
+  the renderer also treats `Some("")` from a caller as `None` so
+  empty-string env vars and missing env vars round-trip the
+  same way.
+
 The shared per-turn plumbing lives in `hive_ag3nt::turn::{write_mcp_config,
 write_settings, write_system_prompt, run_turn, drive_turn,
 emit_turn_end, wait_for_login, compact_session}` so the two binaries