Harden OpenNeuro MEG staging

[FlorianPfaff]

Summary

• Document OpenNeuro workflow staging behavior and dataset-specific safeguards.
• Cap integer PCA component requests to the current training fold.
• Filter unsupported OpenNeuro event rows and drop non-epochable events before staging.

Validation

• git diff --check
• Tests were not run, per request.

[github-actions]

Test Results

    4 files      4 suites   1m 41s ⏱️
  661 tests   652 ✅  9 💤 0 ❌
2 644 runs  2 608 ✅ 36 💤 0 ❌

Results for commit 8636463.

[github-actions]

✅⚠️MegaLinter analysis: Success with warnings

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ ACTION	actionlint	14		0	0	0.51s
✅ COPYPASTE	jscpd	yes		no	no	9.41s
⚠️ MARKDOWN	markdownlint	55	0	1	0	1.83s
✅ PYTHON	ruff	246	0	0	0	0.38s
✅ REPOSITORY	git_diff	yes		no	no	0.03s
✅ YAML	prettier	48	0	0	0	1.72s
✅ YAML	v8r	48		0	0	8.65s
✅ YAML	yamllint	48		0	0	1.28s

Detailed Issues

⚠️ MARKDOWN / markdownlint - 1 error

.github/PULL_REQUEST_TEMPLATE.md:1 error MD041/first-line-heading/first-line-h1 First line in a file should be a top-level heading [Context: "## Summary"]

Notices

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