🔧 chore(standards): sync cpp playbook rules

Vendor standards live under docs/standards/playbook and are synced into .agents/cpp.

Add AGENTS.md entries so agents working in lsp-server follow the C++ ruleset and standards.

AGENTS.md

@@ -0,0 +1,19 @@
+# AGENTS (Repository Rules)
+
+This repository vendors the **playbook** standards snapshot under:
+
+- `docs/standards/playbook/`
+
+Sync language rulesets into the repo root via:
+
+- `sh docs/standards/playbook/scripts/sync_standards.sh cpp` (C++)
+- `sh docs/standards/playbook/scripts/sync_standards.sh tsl` (TSL)
+- `sh docs/standards/playbook/scripts/sync_standards.sh python` (Python)
+
+When working on C++ code, follow the generated ruleset entry:
+
+- `.agents/cpp/index.md`
+
+Human-facing standards snapshot (vendored):
+
+- `docs/standards/playbook/docs/`