test(skills): extract skill discovery tests into an owned test seam
Move the 1,470-line inline tests module out of skills/mod.rs into skills/tests.rs (mechanical motion; tests reference parent items through super:: paths so the move is verbatim apart from one dedent level). The production owner shrinks to 1,346 lines and the counted aggregate drops by the same amount, since tests.rs paths are excluded from the source-structure budget. Verified: skills suites 195/195 PASS from the new location; fmt clean.
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