fix(renovate): validate multiple preset files without multiline step output

[setchy]

Problem

The validator workflow failed on PR #2 (which runs the workflow from main):

##[error]Unable to process file command 'output' successfully.
##[error]Invalid format './renovate-config.json'

The "Determine files to validate" step wrote a newline-separated list to $GITHUB_OUTPUT with echo "files=...". GitHub's file-command format rejects multi-line values written that way — it only accepts the heredoc delimiter form.

Fix

Fold file detection and validation into a single step, passing the file list directly as command arguments. No step output, so no multiline format issue. The auto-detection behaviour is unchanged:

• explicit files input → use it
• repository has renovate.json* / .renovaterc* → validate those
• otherwise (preset repo) → validate all root *.json

Verification

Simulated the exact step logic locally under bash:

• preset-repo auto-detect → validates ./default.json (only preset on main)
• multi-file → Config validated successfully against 2 file(s) (exit 0)

Ordering

Unblocks #2. Should merge before (or with) #2. Draft pending review.