Gate efficiency finding (Low): resolve_skill_catalog only remembered a
shadowed path once it had warned, so a byte-identical (non-symlink) copy
of a higher-tier skill -- e.g. an external_dirs mirror of local skills --
was re-read and SHA-256-hashed against every owner on each catalog
rebuild / prompt-index miss. Key the memo by (shadowed path, *sorted
higher-tier paths) and record it whether or not a warning fires.
Probe: provably_same_skill calls over 5 resolves of one identical
shadowed copy 5 -> 1. Warn-once behaviour is unchanged.
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