docs: scope AGENTS instructions by directory (#2083)

crates/e2e_test/AGENTS.md

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+# E2E Test Crate Instructions
+
+Applies to `crates/e2e_test/`.
+
+## Test Reliability
+
+- Keep end-to-end tests deterministic and environment-aware.
+- Prefer readiness checks and explicit polling over fixed sleep-based timing assumptions.
+- Ensure tests isolate resources and clean up temporary state.
+
+## Environment Safety
+
+- For local KMS-related E2E runs, keep proxy bypass settings:
+  - `NO_PROXY=127.0.0.1,localhost`
+  - clear `HTTP_PROXY` and `HTTPS_PROXY`
+- Do not hardcode machine-specific paths or credentials.
+
+## Scope and Cost
+
+- Place exhaustive integration behavior here; keep unit behavior in source crates.
+- Keep new E2E scenarios focused and avoid redundant overlap with existing suites.
+
+## Suggested Validation
+
+- `cargo test --package e2e_test`
+- Full gate before commit: `make pre-commit`