fix(skills): ignore nested markdown assets in skill_view

[MarsDoge]

Summary

• Fix skill_view("<name>") so nested markdown references/templates/assets inside another skill package are not treated as legacy standalone skills.
• Preserve collision detection for real same-name skills across local/external skill directories.
• Add a regression test covering the bundled-style templates/notion.md asset collision scenario.

Test Plan

• venv/bin/python -m pytest tests/tools/test_skills_tool.py::TestSkillViewCollisionDetection -q
• venv/bin/python -m pytest tests/tools/test_skills_tool.py tests/test_plugin_skills.py -q
• Manual: skill_view("notion") now resolves productivity/notion/SKILL.md successfully.

Closes #35743

[MarsDoge]

Closing as duplicate of #35768, which already fixes #35743 and has passing checks. Thanks!