fix(skills): ignore nested assets in skill lookup

[BROCCOLO1D]

Summary

• Fixes bare skill_view resolution so nested Markdown assets inside another skill are not treated as standalone legacy skill candidates.
• Closes [Bug]: skill_view falsely reports collision when another skill has an asset file with the same name #35743.

Why

• skill_view notion could report an ambiguous collision when another skill contained templates/notion.md or references/.../notion.md.
• Those Markdown files are assets of their parent skill, not loadable skills, so they should not participate in bare-name collision detection.

Changes

• tools/skills_tool.py: skips Strategy 3 legacy <name>.md hits when an ancestor directory already contains SKILL.md.
• tests/test_plugin_skills.py: adds regression coverage for nested asset collisions and preserves top-level legacy flat .md skill lookup.

Validation

• python -m pytest tests/test_plugin_skills.py -q -o 'addopts='
• python -m ruff check tools/skills_tool.py tests/test_plugin_skills.py
• git diff --check

Scope

• In scope: bare-name skill_view candidate discovery for nested Markdown asset files.
• Out of scope: changing explicit categorized skill/file lookup semantics or broader skill index behavior.

[BROCCOLO1D]

Closing this as stale: there has been no activity or maintainer follow-up for over a week, and I don't want to leave inactive work open. Happy to reopen/rework if this becomes useful.