docs(infra): document uv-based Python environment setup (#37881)

Adds an "Environment and dependency management" subsection to the
contributor guidance files, codifying the existing `uv`-only workflow so
agents and contributors don't reach for `pip`/`poetry`/`conda`, pin a
global Python version, or spin up stray virtualenvs. The guidance is
adapted to the monorepo's reality (per-package `requires-python`, `uv
sync`/`uv run` without manual activation) rather than a generic
single-project setup.

AGENTS.md

@@ -73,6 +73,15 @@ make format
 uv run --group lint mypy .
 ```
 
+#### Environment and dependency management
+
+Use `uv` for all environment and dependency operations in this monorepo. Do not invoke `pip`, `poetry`, or `conda` directly.
+
+- Let `uv` manage the interpreter and virtual environments — `uv sync` and `uv run` operate without manual `source .venv/bin/activate`. Do not create ad-hoc virtual environments outside the package directory.
+- Each package targets its own supported Python range via its `pyproject.toml`; do not pin a global Python version. If you need an interpreter explicitly, defer to the package's `requires-python` rather than assuming system Python.
+- Install dependencies explicitly through `uv sync` (optionally `--group <name>` / `--all-groups`); never let them install implicitly.
+- Don't mix environments within a session, and don't add new dependencies unless strictly required — when you do, justify them (recent releases/commits, adoption).
+
 #### Key config files
 
 - pyproject.toml: Main workspace configuration with dependency groups