fix(factory): support '-' as stdin sentinel for --input-file

[flobernd]

Summary

• Treat --input-file - as a request to read JSON input from stdin, matching the Unix convention used by curl, kubectl, jq, and similar tools.
• Previously elastic <cmd> --input-file - failed with Error: --input-file: file not found: - because the resolver passed - to readFileSync unconditionally.
• The fix detects the - sentinel before the file-read branch and delegates to the existing module-level stdinReader; error messages continue to use the --input-file source label for consistency.

Closes #335

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	8.96s
✅ REPOSITORY	gitleaks	yes		no	no	55.12s
✅ REPOSITORY	git_diff	yes		no	no	0.53s
✅ REPOSITORY	secretlint	yes		no	no	30.56s
✅ REPOSITORY	trivy	yes		no	no	18.96s
✅ TYPESCRIPT	eslint	2		0	0	7.01s

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