Enable the pytest plugin from pytest's own config, not only an env var #339

Part of #277

The pytest plugin's only switch is an environment variable. _opted_in() reads
DEVTOOLS_ENABLE/DEVTOOLS_PORT, trace mode reads DEVTOOLS_TRACE, and the
plugin registers no pytest_addoption at all — so there is no way to turn
capture on from the project itself:

DEVTOOLS_ENABLE=1 DEVTOOLS_TRACE=1 pytest        # the only way today

Every pytest plugin offers CLI flags and ini options; an env var is the
fallback for CI, not the interface. It is undiscoverable (pytest --help shows
nothing), uncommittable, and per-shell rather than per-project — a contributor
who clones the repo has no way to see that capture exists.

The opt-in itself is not the problem and must stay. The plugin auto-loads
through an entry point, so installing the package must never change how an
existing suite behaves. A CLI flag or an ini option is equally explicit and
strictly better.

[tool.pytest.ini_options]
devtools = true
devtools_trace = true

pytest --devtools --devtools-trace

Plain scripts already need none of this — devtools.enable(trace=True) is
configured entirely in code, and DEVTOOLS_ENABLE is not consulted for them.
Only the pytest surface is affected.

Worth doing before the first PyPI publish (#294). Changing how capture is
switched on after release is a breaking change for anyone already set up.

Acceptance criteria

pytest --devtools and a [tool.pytest.ini_options] entry each enable capture
with no environment variable, and the same for trace mode. Precedence is CLI →
ini → env. The existing environment variables keep working: CI and the
pnpm demo:python:* scripts use them, and removing them would break setups
that already exist.

Size: S. Blocks #294.