Add try catch around read directories and have new error conditions when files cannot be read, plus test

[reidbaker]

This better handles when skills are symlinked to directories that may not be accessible

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

[gemini-code-assist]

Code Review

This pull request enhances the Validator class to handle FileSystemException and other unexpected errors when reading the SKILL.md file, adding specific error IDs for these scenarios. It also introduces a test case for inaccessible files and refactors path construction to use the path package. The review feedback suggests using _getSeverity for the new error types to allow for severity overrides and recommends wrapping the permission-altering logic in tests within a try-finally block to ensure proper cleanup even if assertions fail.