feat(session-kind): explicit GSTACK_SESSION_KIND override; skill-start spawned gates keyed on kind (#2733)

Claude Code subagents inherit the parent env byte-for-byte, so ambient
markers classify them as the parent's kind and the spawned classification
was unreachable outside OpenClaw. GSTACK_SESSION_KIND=spawned (step 0,
spawned-only by design) lets a dispatching skill mark its subagent per
command. skill-start now keys SPAWNED_SESSION and the spawned-session
instruction block on the resolved kind (was raw OPENCLAW_SESSION),
suppresses CONDUCTOR_SESSION for spawned sessions, gates all 11
interactive-onboarding blocks plus their ack-at-emit marker writes on
kind != spawned, and adds a destructive-gate carve-out to the spawned
block (conservative-continue, never prose-STOP).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

test/preamble-compose.test.ts

@@ -83,6 +83,9 @@ describe('Conductor signal (skill-start script)', () => {
     const script = fs.readFileSync(path.join(import.meta.dir, '..', 'bin', 'gstack-skill-start'), 'utf-8');
     expect(script).toContain('echo "CONDUCTOR_SESSION: true"');
     expect(script).toMatch(/"\$_SESSION_KIND" != "headless"[\s\S]*CONDUCTOR_WORKSPACE_PATH[\s\S]*CONDUCTOR_PORT[\s\S]*CONDUCTOR_SESSION: true/);
+    // #2733: spawned outranks Conductor — a spawned session inside a Conductor
+    // workspace auto-chooses instead of rendering prose to nobody.
+    expect(script).toMatch(/"\$_SESSION_KIND" != "headless"[\s\S]{0,80}"\$_SESSION_KIND" != "spawned"[\s\S]{0,200}CONDUCTOR_SESSION: true/);
   });
 
   test('claude preamble render invokes the script and interprets CONDUCTOR_SESSION', () => {