Use absolute path construction in _process_slce_extra default parameter

[Copilot]

Addresses feedback from #166 to ensure _process_slce_extra constructs paths relative to self.repo_root regardless of execution context.

Changes

• Changed default parameter from filename: str = "packages/matter/matter.slce.extra" to filename: Optional[str] = None
• Construct absolute path inside function body using os.path.join(self.repo_root, "packages/matter/matter.slce.extra") when no filename provided
• Preserve existing behavior when explicit filename is passed (still joined with self.repo_root)

def _process_slce_extra(self, filename: Optional[str] = None) -> dict:
    if filename is None:
        filename = os.path.join(self.repo_root, "packages/matter/matter.slce.extra")
    else:
        filename = os.path.join(self.repo_root, filename)

This eliminates ambiguity around relative path resolution and makes the path construction explicit and deterministic.

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