darnit audit cannot emit SARIF or Markdown from the CLI

[justaugustus]

Description

The documentation advertises SARIF (and Markdown) as output formats, but the darnit audit CLI only offers text and json. SARIF/Markdown are reachable only through the library or MCP tool, so a CLI user following the docs cannot produce SARIF output at all.

Steps to Reproduce

1. Install darnit from source (uv sync).
2. Run darnit audit --help and note the -o/--output choices.
3. Run darnit audit -o sarif <path>.

Expected Behavior

Either sarif and markdown are accepted by -o (as the docs imply), or the docs state clearly that SARIF/Markdown are library/MCP-only.

Actual Behavior

-o accepts only text/json; -o sarif is rejected. Meanwhile the docs advertise SARIF:

• docs/USAGE_GUIDE.md:14 — "Multiple output formats - Markdown, JSON, SARIF"
• docs/USAGE_GUIDE.md:121 — "sarif - GitHub Code Scanning compatible"
• docs/USAGE_GUIDE.md:308 — "Use SARIF output for GitHub Code Scanning"

Root cause:

• packages/darnit/src/darnit/cli.py:908-911 hardcodes -o/--output choices=["text", "json"].
• cli.py:236-241 dispatches only json-vs-text.
• The formatters already exist but only on the library/MCP path: packages/darnit/src/darnit/tools/audit.py:209 (output_format = "markdown"|"json"|"sarif") and packages/darnit/src/darnit/server/tools/builtin_audit.py:37.

Environment

• darnit version: main @ 86f9cfa
• Python version: 3.12 (uv-managed)
• OS: macOS (platform-independent)

Additional Context

Non-prescriptive suggestion: surface the already-advertised formats through the existing -o mechanism (extend choices + dispatch, reusing the existing formatters), optionally adding --output-file — i.e. match the existing output contract rather than introduce a new structure. Alternatively, correct the docs. Related: no documented recipe currently exists for producing a report file from a terminal run; that resolves naturally if the CLI can emit these formats.