v6.1.0 — two skills nobody ran

Removed

• /memory-kit:memory-lint. It linted [[wikilink]] hygiene — broken links, orphan pages,
  missing backlinks — for a convention the kit does not actually ship: nothing in the templates
  emits wikilinks. It sat behind a cp step from v4.2.0 to v6.0.0 and never produced a report in
  this repository's history. What was load-bearing in it (frontmatter coverage, references that
  no longer resolve) the system-audit collector already gathers generically.

Changed

• /memory-kit:memory-usage folded into /memory-kit:system-audit. The transcript profiler
  is real signal — it is the only thing that can answer "did this rule / skill / agent ever
  actually fire?" — but that question belongs to the audit's layer-telemetry lens, which until
  now described a telemetry table nobody generated. The script moved to
  skills/system-audit/scripts/usage.py and the lens invokes it. One fewer skill, one lens that
  can finally be executed instead of aspired to.

The subtraction rule this follows: a layer that has never fired is not "available", it is
decoration. It was written into the audit's own lens 7 before it was applied here.