fix(#913): guard top-level Claude Code plan-phase against role collapse (#915)

Three-part fix for the top-level inline collapse bug:

1. plan-phase.md: add <runtime_compatibility> block after
   </available_agent_types> that makes the Agent-availability
   requirement explicit; workflow fails-closed (stops with a clear
   log) in genuinely Agent-less contexts.

2. plan-phase.md: rename 7 "ORCHESTRATOR RULE — CODEX RUNTIME"
   labels to "ALL RUNTIMES" so the spawn guard applies universally
   (not just when Codex is detected).

3. execute-phase.md: scope the existing "Other runtimes" inline-
   fallback prose to non-Claude contexts, preserving the #853
   backgrounded-agent behaviour for Claude Code background agents.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
(cherry picked from commit 5bf77a527f)

gsd-core/workflows/execute-phase.md

@@ -14,8 +14,11 @@ Orchestrator coordinates, not executes. Each subagent loads the full execute-pla
   instead of spawning parallel agents. Only attempt parallel spawning if the user
   explicitly requests it — and in that case, rely on the spot-check fallback in step 3
   to detect completion.
-- **Other runtimes:** If `Agent`/`agent` tool is unavailable, use sequential inline execution as the
-  fallback. Check for tool availability at runtime rather than assuming based on runtime name.
+- **Other runtimes:** If `Agent`/`agent` tool is genuinely unavailable (e.g. a backgrounded
+  Claude Code agent per #853, or a non-Claude runtime), use sequential inline execution as
+  the fallback for executor parallelization only. If `Agent` IS available (top-level Claude
+  Code), you MUST spawn gsd-executor agents — inline execution is not authorized. Check for
+  actual tool availability, not runtime name.
 
 **Fallback rule:** If a spawned agent completes its work (commits visible, SUMMARY.md exists) but
 the orchestrator never receives the completion signal, treat it as successful based on spot-checks