perf(prompt): cache kanban worker guidance at session init

Salvages #24402 by @RyanRana. The KANBAN_GUIDANCE block (~835 tokens)
is session-static — the dispatcher decides at spawn time whether the
process is a kanban worker via the kanban_show tool's check_fn (gated
on HERMES_KANBAN_TASK env var). Re-checking 'kanban_show' in
valid_tool_names and re-loading the reference on every system-prompt
rebuild (init + each context compression) is wasted work.

Caches the resolved string on agent._kanban_worker_guidance once in
agent_init and consumes it in system_prompt.build_system_prompt(),
with a getattr fallback for code paths that bypass agent_init.

agent/agent_init.py

@@ -828,7 +828,6 @@ def init_agent(
         tool_names = sorted(agent.valid_tool_names)
         if not agent.quiet_mode:
             print(f"🛠️  Loaded {len(agent.tools)} tools: {', '.join(tool_names)}")
-            
             # Show filtering info if applied
             if enabled_toolsets:
                 print(f"   ✅ Enabled toolsets: {', '.join(enabled_toolsets)}")
@@ -836,7 +835,18 @@ def init_agent(
                 print(f"   ❌ Disabled toolsets: {', '.join(disabled_toolsets)}")
     elif not agent.quiet_mode:
         print("🛠️  No tools loaded (all tools filtered out or unavailable)")
-    
+
+    # Kanban worker/orchestrator lifecycle guidance is session-static:
+    # the dispatcher decides at spawn time whether this process is a kanban
+    # worker (kanban_show tool is present iff HERMES_KANBAN_TASK is set).
+    # Resolving the ~835-token block once here avoids re-running the
+    # membership test + reference on every system-prompt rebuild
+    # (init + each context compression).
+    from agent.prompt_builder import KANBAN_GUIDANCE
+    agent._kanban_worker_guidance = (
+        KANBAN_GUIDANCE if "kanban_show" in agent.valid_tool_names else ""
+    )
+
     # Check tool requirements
     if agent.tools and not agent.quiet_mode:
         requirements = _ra().check_toolset_requirements()