feat(probes): add CSV/spreadsheet formula injection probe and detector

[eddieran]

What

Adds probes.formula_injection.FormulaInjection and a matching detectors.formula_injection.FormulaInjection covering CSV / spreadsheet formula injection (CWE-1236 / OWASP CSV Injection), plus probe and detector docs pages.

The probe elicits CSV cell values that begin with a formula trigger (=, +, -, @) and carry a DDE / shell / exfiltration payload. The detector flags any newline- or comma-delimited cell that begins with a trigger and contains a dangerous token, so benign formulas such as =SUM(A1:A2) do not score — deterministic, no judge/model cost.

This treats the spreadsheet as a downstream consumer of target output, alongside ansiescape (terminal) and web_injection (browser).

Not a duplicate

Scoping issue: #1906. No existing or open PR touches CSV / formula / spreadsheet injection. Distinct from the in-flight exploitation injection work (#1870 OS-command, #1871 NoSQL), which injects into code interpreters the target feeds; this is a separate downstream consumer (a spreadsheet), shipped as its own module. Happy to fold into exploitation if the maintainers prefer.

Verification

python -m pytest tests/probes/test_probes_formula_injection.py tests/detectors/test_detectors_formula_injection.py   # 7 passed
python -m pytest tests/test_docs.py tests/plugins/test_plugins.py                                                     # 673 + 361 passed
python -m garak -t test.Blank -p formula_injection -d always.Pass                                                    # runs clean, 40 prompts

AI assistance

Written with AI assistance (Claude Code). I'm the submitter and take responsibility for the change.