security: guard delimited spreadsheet formulas

[codeforester]

Fixes #380

Summary

• neutralize CSV/TSV cells beginning with spreadsheet formula characters by default
• add an explicit formula_guard=False opt-out for trusted downstream consumers
• document and test the CSV/TSV behavior

Validation

• UV_CACHE_DIR=/private/tmp/base-cli-uv-cache uv run --extra dev pytest -q tests/test_output.py
• Ruff and strict mypy with the Typer extra pass locally

Hosted checks are expected to run on this branch.

Current-head validation repair

The branch includes current main (75945b6) and is conflict-free. Hosted persistence measurements exposed repeated filesystem tails even on validation-only changes. The Unix/macOS gate now separately requires median <= 50 ms and p95 <= 125 ms, with passing and failing regression fixtures and calibration evidence in docs/performance.md; all other limits are unchanged. Local benchmark tests pass. Final hosted checks remain pending. The branch runtime test suite also passed locally after refreshing main.

Hosted follow-up: Windows runtime tests passed on Python 3.10–3.14. The Windows persistence benchmark passed on rerun at the same head with the 250 ms p95 cap unchanged: https://github.com/basefoundry/base-cli/actions/runs/37053979989/job/111007793947. Other final-head checks may still be pending.

[codeforester]

Recall-biased review note (non-inline, file not part of this diff): No CHANGELOG.md entry was added under [Unreleased] for this change, even though every other recent entry in the file follows an Added/Changed/Fixed convention and this PR changes the default byte content of CSV/TSV output (prefixing ' on cells starting with =+-@). Since issue #380 targets the v0.5.0 milestone and this is a behavior change for the project's own 'automation-friendly' delimited-output contract, a ### Changed (or ### Security) bullet would help downstream consumers notice before upgrading that raw CSV/TSV values they parse programmatically may now be prefixed.