docs: accept several skill names in skills:eval and re-aim overlapping cases

Adds a multi-name filter to `npm run skills:eval` so a set of related
skills can be iterated together, and makes an unknown name a hard error
rather than an empty run — a typo would otherwise report a green 0/0
that reads exactly like a clean pass of the skill that was meant.

Re-aims two `local-dev` cases and one `test-servers` case that partially
overlapped `AGENTS.md`. Partial overlap oscillates across runs rather
than sitting at zero, so it reads as eval noise instead of a defect in
the case; `docs/skill-authoring.md` now records that failure mode and
the observed scores.

Widens the `test-servers` description to the "Use when …" shape and
points the `testing` skill at it, since anything needing a real server
to run against is a procedure `testing` does not carry. That growth
moves the skill listing from 2,921 to 3,144 of 4,000 characters, so the
figure recorded in AGENTS.md is updated to match.

Signed-off-by: cliffhall <cliff@futurescale.com>

AGENTS.md

@@ -233,7 +233,7 @@ that from happening:
    overflows, and drops the least-invoked entries **first** — which are exactly the
    model-invoked skills that must fire on their own. `verify:skills` prints the
    current cost against the budget recorded in `scripts/lib/skill-manifest.mjs`
-   (2,921/4,000 characters as of this writing) and fails when it is exceeded. Raise
+   (3,144/4,000 characters as of this writing) and fails when it is exceeded. Raise
    the budget deliberately, or tighten a description; each entry is capped at 1,536
    characters regardless, so **put the key use case first**.