Document lint command and pre-commit hook

[vl3c]

Summary

The pre-commit hook (hooks/pre-commit) and the test lint CLI command weren't mentioned in any docs; only the code referenced them. This PR adds:

• cli/README.md
  • A new Linting section: usage, all options (checked against test lint --help), and what the hook does, including the main-repo venv lookup for worktrees added in Find main repo venv in pre-commit hook when run from a worktree #53.
  • A note that the installed hook is a copy and must be reinstalled when hooks/pre-commit changes.
  • Linting added to the Overview and to the tests.py architecture notes.
• README.md
  • An optional step 5 in §4.2 Environment Setup that installs the hook.
  • A linting entry in §6.8 Testing.

Note

--install-hook currently fails with FileNotFoundError from a linked worktree: it writes to <worktree>/.git/hooks/, but in a worktree .git is a file. For now the docs say to run it from the main checkout. A code fix can follow in a separate PR, which would drop that caveat.

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