✨ feat(plugin): add --pytest-env-verbose for debugging env assignments

[gaborbernat]

When multiple env files, inline config, CLI --envfile options, and exported variables all interact, tracking what pytest-env actually sets becomes painful. This is especially true after the recent addition of .env file loading and the --envfile CLI option, which significantly increased the number of possible sources. Closes #198.

The new --pytest-env-verbose flag prints every environment variable action (SET, SKIP, UNSET) along with its source file in the test session header via pytest_report_header, following the same pattern pytest's own cacheprovider uses. This avoids the get_terminal_writer() assertion error that occurs during early hooks and integrates cleanly with pytest's output capturing.

Example run with a pyproject.toml config, a .env file, and TEMP_VAR exported in the shell:

$ pytest --pytest-env-verbose
============================= test session starts ==============================
platform darwin -- Python 3.14.3, pytest-9.0.2, pluggy-1.6.0
pytest-env:
  SET   API_KEY=secret123  (from /project/.env)
  SET   DEBUG=true  (from /project/.env)
  SET   DATABASE_URL=postgres://localhost/test  (from /project/pyproject.toml)
  SKIP  HOME=/Users/me  (from /project/pyproject.toml)
  UNSET TEMP_VAR  (from /project/pyproject.toml)
rootdir: /project
configfile: pyproject.toml
collected 1 item

test_it.py .                                                             [100%]
============================== 1 passed in 0.01s ===============================

This PR also replaces prettier with mdformat (with mdformat-toc, mdformat-gfm, and mdformat-config plugins) and yamlfmt in pre-commit, and restructures the README for readability within the existing Diataxis layout -- grouping CLI options together, consolidating small how-to guides, and adding an auto-generated table of contents.