Add missing linter hooks from claude-config-template

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Summary

Adds five new linter hooks that were missing from git-hooks compared to
claude-config-template/check_scripts/:

• codespell — typo detection (simple/check_codespell{,_all}.sh)
• gitleaks — secret/token scanner (git/check_gitleaks{,_all}.sh, optional)
• json validation — via python3 -m json.tool (simple/check_json_validate{,_all}.sh)
• ruff — modern Python linter (python/check_ruff{,_all}.sh, coexists with flake8/mypy)
• shfmt — shell formatter with -i 2 -ci (shell/check_shfmt{,_all}.sh)

Each hook has a dedicated bats-style test suite under tests/ with four cases:
positive, negative, no-matching-files, binary-missing. All 24 suites pass via
./tests/run_all.sh.

CI (.github/workflows/tests.yml) gains simple-tests, git-tests,
shell-tests jobs plus install steps for ruff and codespell.
README.md and CLAUDE.md document the new hooks and directory layout.

Also:

• Adds tasks/ to .gitignore so local task specs don't pollute commits.
• Out of scope (per task): Docker variants, pre-commit framework, default
  configs, removal of flake8/mypy.

Closes #16