[FEA]: Machine-readable catalog fleet rollup report

[mimran-khan]

Problem to solve

Catalog validate writes per-skill reports under reports/<skill>/, but the only fleet-level output is the Rich terminal scoreboard. Registry CI and org merge gates need a single machine-readable rollup with per-skill status, severity totals, and pointers to child reports.

Proposed approach

After catalog validation finishes, write catalog-summary.json at the reports root with total, passed, failed, and a skills[] array (name, pass/fail, reason, optional severity data from per-skill JSON when present). Keep the terminal scoreboard for humans.

Additional context

Complements per-skill BENCHMARK.md work but targets fleet aggregation. Validated gap on main; no duplicate open issue found.

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