Fix duplicate guidelines when using symlinked paths

[damianlewis]

Summary

When .ai/guidelines is a symlink (or contains symlinked files), GuidelineComposer produces duplicate guidelines in the generated output. This happens because realpath() is applied inconsistently — Symfony Finder resolves symlinks via getRealPath(), but customGuidelinePath() returns unresolved paths. The deduplication logic then fails because it compares these mismatched path strings.

This PR applies realpath() to all paths derived from customGuidelinePath() so that path comparisons and deduplication work correctly regardless of whether the guidelines directory or individual files are symlinked.

Benefit to end users: Users who symlink their .ai/guidelines directory (e.g. to an external repo shared across projects) no longer see duplicate guideline sections in their generated agent files.

Does not break existing features: realpath() is a no-op on non-symlinked paths, so behaviour is unchanged for users with regular directories. All existing tests continue to pass.

Changes

• Resolve customGuidelinePath() before str_contains comparisons in guidelinePath() and guideline()
• Resolve paths returned from user override detection in guidelinePath()

Test plan

• New test: symlinked custom guidelines directory does not produce duplicates
• New test: symlinked custom guideline file does not produce duplicates
• Both tests fail without the fix (asserting 2 is not 1) and pass with it
• All existing tests pass
• Pint, PHPStan, and Rector checks pass
• Verified fix in a real project with a symlinked .ai/guidelines directory