feat: add code formatting and linting pipeline

[aksg87]

Description

Introduces automated code formatting and linting to ensure consistent style across LangExtract. Uses pyink (Google's Black fork), isort, and pylint following Google's Python Style Guide.

Note: This PR uses three commits instead of squashing to one because we need CI to stay green and isolating the formatting changes is useful for clarity. Can't add format checks before formatting the code (would fail), and can't format without the config first. So:

Three commits:

1. Configuration only - Add formatter/linter configs and autoformat.sh script
2. Apply formatting - Mechanical changes only, no functional modifications
3. Enable CI checks - Activate format/lint environments in tox

Type: Code health

How Has This Been Tested?

$ tox  # All tests pass on Python 3.10/3.11, format and lint checks pass
$ ./autoformat.sh  # Script works correctly
$ pre-commit run --all-files  # All hooks pass

Checklist:

• I have read and acknowledged Google's Open Source
  Code of conduct.
• I have read the
  Contributing
  page, and I either signed the Google
  Individual CLA
  or am covered by my company's
  Corporate CLA.
• I have discussed my proposed solution with code owners in the linked
  issue(s) and we have agreed upon the general approach.
• I have made any needed documentation changes, or noted in the linked
  issue(s) that documentation elsewhere needs updating.
• I have added tests, or I have ensured existing tests cover the changes
• I have followed
  Google's Python Style Guide
  and ran pylint over the affected code.