Hide "Mark as Viewed" checkboxes on files under commit nodes

[Copilot]

Files under individual commits in the "Commits" section displayed "Mark as Viewed" checkboxes. These checkboxes should only appear on files in the top-level "Files" section, as the viewed state applies to the current file version, not historical snapshots.

Changes

• FileChangeNode.checkboxState: Made optional to support files without checkboxes
• FileChangeNode.isUnderCommitNode(): Added helper that walks parent hierarchy to detect if node is under a CommitNode (identified by contextValue === 'commit')
• FileChangeNode.updateViewed(): Conditionally sets checkboxState to undefined for files under commits
• DirectoryTreeNode.allChildrenViewed(): Handle optional checkboxState when computing folder checkbox state
• TreeUtils.processCheckboxUpdates(): Skip files without checkboxState when processing bulk checkbox changes

Behavior

Before: Files under commits show checkboxes; checking them marks the file as viewed globally
After: Only files in the top-level "Files" section show checkboxes

Original prompt

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This section details on the original issue you should resolve

<issue_title>Mark as Viewed: Marking a file as views on an older commit marks the file as viewed for all versions of the file</issue_title>
<issue_description></issue_description>

<agent_instructions>Don't show the mark as viewed checkboxes on files under commit nodes.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Mark as Viewed: Marking a file as views on an older commit marks the file as viewed for all versions of the file #8313

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