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>

.claude/skills/testing/SKILL.md

@@ -11,6 +11,12 @@ statements, functions, and branches.** That rule and the React/Mantine
 conventions live in [`AGENTS.md`](../../../AGENTS.md); this skill is where a
 test goes, how to run it, and how to clear the gate.
 
+⚠️ **Anything that needs a real server to run against — an integration test, a
+smoke, reproducing a bug by hand — is `/test-servers`, and you have to load it.**
+Integration and smoke tests here drive a real server over a real transport
+rather than a mock, so picking, building and connecting to a fixture is a
+procedure of its own that this skill does not carry.
+
 ## Where the test file goes
 
 **Side-by-side by default; `src/test/` only for what can't be co-located; and