PYTHON-5768 Update Copilot instructions with new guidelines

Added guidelines for security, performance, documentation, and review style.

Author: aclark4life

.github/copilot-instructions.md

@@ -1,3 +1,8 @@
-Please see [AGENTS.md](../AGENTS.md).
+Follow these rules in this repo (summary). Full details: see `AGENTS.md` in the repository root.
 
-Follow the repository instructions defined in `AGENTS.md` when working in this codebase.
+- Security: never introduce hardcoded secrets; avoid injection/dynamic execution hazards.
+- Performance: watch for inefficient loops and resource leaks; clean up resources.
+- Docs/API: public classes/modules/methods must have Sphinx docs.
+- PyMongo synchro rule: do not edit `pymongo/synchronous` (or sync test twins) unless the change includes `_IS_SYNC`; those files are generated via `tools/synchro.py`.
+- Async: async functions must not call blocking I/O.
+- Review style: be specific/actionable; explain “why”; ask clarifying questions when unclear.