Support internal: true metadata to skip installing certain skills

[jakemac53]

Closes #502

• Adds a simple parser for skill frontmatter as well.
• Uses that to extract the internal metadata field if present and skip skills unless the environment variable is set
• Cleans up some older tests that I noticed were set up wrong or invalid.

[github-actions]

Package publishing

If you have publishing permissions, you can use the links below to publish the changes after merging this PR.

Package	Version	Status	Publish tag (post-merge)
package:dart_mcp	0.5.2	already published at pub.dev
package:dart_mcp_server	1.1.0-wip	WIP (no publish necessary)
package:skills	0.4.0-dev	ready to publish	skills-v0.4.0-dev

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.

[gemini-code-assist]

Code Review

This pull request adds support for filtering internal skills by parsing internal: true metadata from the frontmatter of skill files, skipping them during scanning unless the INSTALL_INTERNAL_SKILLS=1 environment variable is set. The feedback suggests optimizing frontmatter parsing to avoid unnecessary string allocations and recommends using local parameters or fields instead of directly referencing the global environment variable check within the scanners.

[github-actions]

PR Health

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