Add OAR implementation guardrails

Author: johnnygreco

projects/openshell-agent-runner/AGENTS.md

@@ -7,6 +7,17 @@
   semantics wherever OAR exposes OpenShell behavior. Introduce OAR-specific
   terms only for behavior that OAR owns.
 
+## Implementation
+
+- Prefer the smallest direct change that satisfies a concrete requirement. Do
+  not add abstractions, extension points, compatibility layers, or fallbacks for
+  hypothetical future needs.
+- Keep defensive programming proportionate to realistic failures. Validate
+  external inputs and trust boundaries, but do not complicate internal code for
+  implausible states already constrained by the system.
+- Do not expand the task to adjacent cleanup, features, or policy changes unless
+  they are required to deliver the requested outcome.
+
 ## Developer workflow
 
 - Run focused tests while iterating. Before handoff, run `make check` and