docs: address comprehensive review feedback

- Add precedence hierarchy for conflict resolution
- Clarify multi-directory change handling
- Add README/docs links for new contributors
- Add explicit test coverage and regression requirements
- Define user-visible changes with examples
- Add validation workflow guidance

Author: napetrov

.github/copilot-instructions.md

@@ -7,20 +7,34 @@ For project context and directory routing, read root `AGENTS.md` first.
 
 `AGENTS.md` files provide directory scope/guardrails and should not duplicate Copilot policy text.
 
+## Precedence hierarchy (conflict resolution)
+When guidance conflicts across files, apply in this order:
+1. `.github/copilot-instructions.md` (this file)
+2. Applicable `.github/instructions/*.instructions.md` files
+3. Nearest local `AGENTS.md` for edited path
+4. Root `AGENTS.md`
+
 ## Mandatory flow
 1. Read root `AGENTS.md`.
-2. Read nearest local `AGENTS.md` for edited files.
-3. Most specific AGENTS wins when multiple apply.
+2. For each edited file, read its nearest local `AGENTS.md`.
+3. If multiple directories changed, apply each scope's local AGENTS to corresponding files.
+4. Most specific AGENTS wins for each file.
 
 ## Contribution expectations
 - Keep diffs minimal and scoped to the task.
 - Preserve NumPy-compatible API behavior by default.
-- For behavior/API changes: update tests, and docs/examples when user-visible.
-- For bug fixes: prefer adding regression coverage.
+- For behavior/API changes: update tests, and docs/examples when user-visible (API signature/behavior, errors, examples output).
+- For bug fixes: add regression test coverage.
+
+## Test and validation requirements
+- Add unit/integration tests for changed code paths.
+- For bug fixes: include regression tests.
+- Keep tests deterministic and scoped to changed behavior.
 
 ## Authoring rules
 - Use source-of-truth files for mutable details.
 - Do not invent or hardcode versions, flags, or matrix values.
+- Prefer single-purpose commits; avoid opportunistic refactors.
 
 ## Source-of-truth files
 All paths relative to project root:

.github/instructions/testing.instructions.md

@@ -4,6 +4,17 @@ applyTo: "{dpnp/tests/**,tests_external/**}"
 
 # Testing Instructions for GitHub Copilot
 
-- Add regression tests for bug fixes.
+## Test coverage requirements
+- Add unit/integration tests for all changed code paths.
+- For bug fixes: include regression tests that cover the fixed scenario.
+
+## Test quality
 - Keep tests deterministic and scoped to changed behavior.
 - Prefer smallest test surface proving correctness.
+
+## Validation workflow
+Ensure tests pass locally before PR submission:
+```bash
+pytest dpnp/tests/        # core tests
+pytest tests_external/    # integration tests
+```

AGENTS.md

@@ -5,6 +5,10 @@ Entry point for agent context in this repo.
 ## What this repository is
 `dpnp` is the Data Parallel Extension for NumPy: a NumPy-compatible Python API with accelerated execution backends and cross-platform packaging/CI.
 
+For detailed documentation, build instructions, and contribution guidelines, see:
+- `README.md` (project overview and quickstart)
+- `CONTRIBUTING.md` (if exists, or refer to project docs)
+
 ## How to work in this repo
 - Keep changes small and single-purpose (bug fix, perf, docs, or infra).
 - Preserve NumPy-compatible API behavior by default; call out intentional behavior changes in PR text.