docs: add repo-local AGENTS.md contributor guidance

AGENTS.md

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+# AGENTS Instructions
+
+This file provides machine-readable contributor guidance for CrewAI.
+
+## Scope
+
+- Keep changes focused to one behavior contract per PR.
+- Prefer deterministic behavior and fail-closed defaults for tool/permission boundaries.
+- Do not bundle unrelated refactors.
+
+## Setup
+
+- Python: `>=3.10,<3.14`
+- Package manager: `uv`
+- Create environment from repo root:
+  - `uv venv`
+  - `uv sync`
+
+## Validation (repo-local baseline)
+
+Run these from `lib/crewai/` unless otherwise noted.
+
+1. Format/lint before tests:
+- `uv run ruff check .`
+- `uv run ruff format .`
+
+2. Type checks:
+- `uvx mypy src`
+
+3. Tests:
+- `uv run pytest .`
+
+4. Build sanity (if packaging changes):
+- `uv build`
+
+## Engineering Guardrails
+
+- Preserve deterministic crew/task execution order; add regression tests for ordering and retry semantics.
+- Keep tool execution and permission behavior explicit; prefer hard failure over permissive fallback on ambiguous safety state.
+- For concurrency changes, include tests that verify stable ordering or explicit ordering metadata.
+
+## PR Hygiene
+
+- PR description must include: Problem, What changed, Validation commands + results.
+- Keep branch scope aligned to issue scope.
+- If scope changes, update PR title/body immediately.
+