Strengthen skill guidance on submodule merge sequence

[mattleibow]

This pull request updates the documentation and submodule handling process to prevent critical issues when merging native dependency updates, specifically around squash-merging and submodule references. It also updates the externals/skia submodule to a new commit.

Critical documentation updates to merge process:

• Added a warning to .github/skills/native-dependency-update/SKILL.md about the dangers of merging both PRs without updating the submodule in between, highlighting that squash-merging creates a new SHA and can orphan commits, breaking users if the submodule isn't updated.
• Expanded the documentation with a detailed, step-by-step mandatory merge sequence, emphasizing that the submodule must be updated to the new squashed commit before merging the SkiaSharp PR, and added explicit warnings against improper merge order.

Submodule update:

• Updated the externals/skia submodule to point to commit 346cbd67e038d3b432b08a89316527625cd30ecc.

[github-actions]

Triage Summary

Labels will be applied specifically for documentation updates related to the merging process and submodule handling.

This issue is not a regression as it pertains solely to documentation updates and does not involve any specific backend technologies or operational systems.

Additional remarks:

• The issue does not pertain to any specific operating system or backend technologies, which limits the applicability of further labels.
• It primarily addresses the merging process and submodule handling without inaccuracies in performance, reliability, or compatibility.

Detailed Summary and Actions

Summary of the triage:

• The issue focuses on documentation updates related to the merging process and submodule handling.
• There are no specific details regarding the operating system or backend technologies discussed.
• The issue does not suggest any regression, as it is purely documentation based.

Summary of the actions that will be performed:

Action	Item	Description
Apply Label	area/Docs	The issue relates to updates in documentation regarding merging and submodules.

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