fix: protect externally managed skills - #2371

Compared 3b243d644ef6b92508cb656405bf6b5a9f8172da (base) against a259c638f5f352e9d58872981cb10d9c0059b406 (head).

• graphify/\_\_main\_\_\.py · _check_skill_version: unsupported — parameter `skill_dst` is annotated `Path` — outside the synthesizable primitive/collection set
• graphify/install\.py · _copy_skill_file: unsupported — not verifiable: all 9 sampled inputs raised on both versions — the function never executed, so 'no divergence' would be vacuous (mostly KeyError — names the real obstacle, not a sampling gap)
• graphify/install\.py · _remove_skill_file: unsupported — not verifiable: all 9 sampled inputs raised on both versions — the function never executed, so 'no divergence' would be vacuous (mostly KeyError — names the real obstacle, not a sampling gap)

Backend availability notices:

• none of the 0 changed id(s) matched a code node in the graph (0 known) — 5 changed function(s) recovered directly from the git diff instead (graph may be stale/unsynced; consider a rebuild)

Formal verification is advisory: equivalent is a proof over a bounded sound subset; may_equivalent is sampled, not proven; unsupported and error are honest abstentions.