fix(tree): preserve mixed-case slugs

[perber]

Allow manually set mixed-case slugs while keeping generated slugs lowercase. Preserve filesystem slug casing during reconstruct to avoid mismatches after rescans.

[Copilot]

Pull request overview

This PR updates slug handling to preserve mixed-case slugs when explicitly set (and when reconstructing from the filesystem), while keeping generated slugs normalized.

Changes:

• Expanded slug validation to accept uppercase letters while keeping reserved-slug detection case-insensitive.
• Updated filesystem tree reconstruction to keep the on-disk slug casing instead of normalizing directory/file names.
• Added tests covering uppercase/mixed-case slug validation, update behavior, and reconstruct behavior.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
internal/wiki/wiki_test.go	Adds coverage ensuring UpdatePage accepts and preserves a mixed-case slug.
internal/core/tree/slug_service_test.go	Adds a test confirming IsValidSlug permits uppercase letters.
internal/core/tree/slug_service.go	Adjusts slug validation rules to allow uppercase and improves docstrings.
internal/core/tree/node_store_reconstruct_test.go	Updates reconstruct tests to reflect “preserve casing” behavior and adds a mixed-case filename test.
internal/core/tree/node_store.go	Changes reconstruct logic to validate and use filesystem names directly (preserving casing).

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