Windows: reject absolute and malformed paths in git tree writes

[pjbgf]

On Windows, absolute paths (e.g., /C:/Users/...) could leak into ApplyTreeChanges, producing empty-named tree entries when split on "/". Add normalizeGitTreePath to validate tree paths are relative with no empty segments, and normalizeRepoRelativeTreePath to convert absolute paths to repo-relative before tree construction.

Fixes #886.

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Note

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[Copilot]

Pull request overview

This PR hardens git-tree construction in the checkpoint/shadow-branch write path by ensuring only valid, repo-relative tree paths are used, preventing malformed entries (notably on Windows when absolute paths leak in).

Changes:

• Added normalizeGitTreePath and integrated it into ApplyTreeChanges and BuildTreeFromEntries to reject absolute paths and invalid segments (empty, ., ..).
• Added normalizeRepoRelativeTreePath to convert absolute in-repo filesystem paths back to repo-relative tree paths before building changes.
• Added tests to ensure invalid paths are skipped and no empty-named tree entries are produced.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
cmd/entire/cli/checkpoint/temporary.go	Normalizes modified/deleted paths to repo-relative tree paths before applying tree surgery; normalizes metadata dir path passed into tree construction.
cmd/entire/cli/checkpoint/parse_tree.go	Adds path normalization/validation and applies it inside ApplyTreeChanges; adds helpers to detect absolute paths and log skips.
cmd/entire/cli/checkpoint/parse_tree_test.go	Adds coverage asserting invalid paths are skipped and tree entries never have empty names.
cmd/entire/cli/checkpoint/checkpoint_test.go	Adds end-to-end-ish coverage for WriteTemporary path normalization (absolute in-repo -> relative) and invalid path skipping.