fix(cli): write concatenated multi-skill report to --output for non-JSON formats

[rodboev]

Summary

skillspector scan ./skills/ --recursive --format markdown --output report.md prints to stdout instead of writing report.md. The JSON recursive path and single-skill mode both write the file correctly; only the recursive + non-JSON combination silently ignores --output.

Closes #203

Root cause

_scan_multi_skill() (cli.py:434-437) passes None instead of output to _write_result(), so _write_result() routes to stdout unconditionally. A naive None → output substitution would also fail: _write_result calls Path(output).write_text(...) per skill, truncating the file on each call and leaving only the last skill's report.

Diff Notes

• src/skillspector/cli.py: add private _result_body() helper (extracts report_body or SARIF fallback); refactor _write_result body resolution to use it (no behavior change); rewrite cli.py:434-437 to concatenate per-skill bodies with --- {skill.relative_path} --- separators and write once via Path(output).write_text(); file is always created when -o is given. JSON branch (lines 410-433) and single-skill path (line 321) untouched.
• tests/unit/test_cli.py: two new tests — one asserts the file is created with both skills' content and stdout is not the destination; one locks the JSON branch behavior.

Scope

No change to the JSON multi-skill branch, single-skill path, _scan_multi_skill() signature, or summary-table stdout output. No new dependencies. Concatenated non-JSON output is not valid SARIF; this matches the issue's Option 1 scope.

Before / After

Invocation	Before	After
scan ./skills/ --recursive --format markdown --output report.md	report.md not created; output to stdout	report.md written with per-skill sections
scan ./skills/ --recursive --format json --output combined.json	combined.json written (unchanged)	combined.json written (unchanged)
scan ./skill --format markdown --output report.md	report.md written (unchanged)	report.md written (unchanged)

Verification

• .\.venv\Scripts\python.exe -m pytest tests/unit/test_cli.py -v — 9 passed, 0 failed (7 pre-existing + 2 new)
• uv run ruff check src/skillspector/cli.py tests/unit/test_cli.py — exit 0
• uv run ruff format --check src/skillspector/cli.py tests/unit/test_cli.py — 2 files already formatted, exit 0
• CI Lint & Test (Python 3.12), Lint & Test (Python 3.13), and DCO Check — pending maintainer approval if fork checks are gated

Notes

DCO sign-off required on every commit (git commit -s). Known existing failures (tests/nodes/test_meta_analyzer.py) are unrelated to this surface.