[raphaelnz]

Why

Protect local edits when maintainers sync upstream. The dirty-tree guard currently exempts paths that share an owned path's prefix, even though the build does not preserve those siblings.

Closes #44.

What changed

• Read NUL-delimited Git status records and compare literal owned paths with directory boundaries.
• Check both sides of renames and retain failure behavior when Git status fails.
• Add isolated sync regression cases, run them in CI, and document the ownership boundary.

Scope

• Covers local-change detection before the skill and agent trees are replaced.
• Does not change sync recovery, release versions, or upstream target selection.

Blast Radius

Sync now rejects dirty nonowned siblings instead of deleting them. Edits within owned paths remain allowed and preserved.

Verification

• A real sync to a synthetic commit based on canonical upstream rejected an untracked prefix sibling with exit 1 and preserved its bytes and the pin.
• A successful sync preserved an owned edit, installed new upstream content, and advanced the pin.
• The new regression script fails against the unfixed guard and passes all 14 cases after the fix. Shell syntax checks and the full port gate also pass.