docs: harden agent parity rule — all platforms in same change, no exceptions

- CODING_GUIDELINES.md: tighten parity rule wording to match Mike's intent:
  "add feature X" means Windows + Linux + macOS in the same commit
- memory: add feedback_gururmm_agent_parity for future session enforcement

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.claude/CODING_GUIDELINES.md

@@ -128,10 +128,10 @@ A feature that silently no-ops on one platform is a gap, not a cross-platform im
 
 ### The rule
 
-> If you add or change a feature in the agent and the change is not blocked by OS-level APIs,
-> you must implement or stub it on all three platforms in the same PR.
-> If a real implementation is not feasible, add a `// TODO(platform): <os> — <reason>` comment
-> and open a tracking item.
+> "Add feature X to the agent" means Windows + Linux + macOS. All three, in the same change.
+> No exceptions for convenience. If a real implementation is not feasible on a given platform,
+> add a working stub and a `// TODO(platform): <os> — <reason>` comment in the same commit.
+> A feature that silently no-ops on one platform without a stub and TODO is a bug, not a gap.
 
 ### cfg gating — choose the right target