docs: address Copilot review round 4 on #2203

Corrects the rationale for the five-positive floor, which was wrong. The
claim was that five prompts make a skill's rate flake-tolerant. The
evaluator scores each prompt on its own `passes / RUNS`, so the number of
prompts never enters any rate and cannot affect a threshold crossing —
`RUNS` is the only knob that does. The floor is worth keeping for
breadth: one prompt measures one phrasing, and a description that fires
on a single narrow shape would pass on its one lucky case.

The bad reasoning had reached five places — AGENTS.md, the authoring
checklist, the JSDoc, the check-site comment and the regression test —
plus the emitted message, which asserted it too. All corrected.

Moves `MIN_POSITIVE_CASES` above `validateEvalCases`'s JSDoc. It had been
inserted between the two, so the `@param`/`@returns` block documented the
constant and the function had none.

Replaces the two remaining cases answerable from `AGENTS.md`: `local-dev`'s
web-client-startup case (the rules file gives the exact command) and
`project-structure`'s clients/web-versus-core case (its tree answers it).
Both replacements probed 3/3 and measure 100%.

RUNS=5 skills:eval: 63/63, every local-dev and project-structure case at
100%. local:gate passes, 34/34.

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

AGENTS.md

@@ -223,9 +223,10 @@ that from happening:
    `evals/evals.json` — positives **and negatives**. Seeing a skill fire once tells
    you Claude found it, not that it finds it reliably, and a skill that fires on
    everything is a context regression that nobody notices by hand. `verify:skills`
-   requires both kinds — and at least **five** positives, since at `RUNS=5` a
-   skill measured by one or two prompts reports a rate a single flake can move
-   across the threshold; `npm run skills:eval` actually runs them (it needs the
+   requires both kinds — and at least **five** positives, for breadth rather than
+   variance: each prompt is scored on its own `passes / RUNS`, so more prompts
+   steady nothing, they cover more of the ways someone might reach the skill and
+   expose a description that only fires on one narrow phrasing; `npm run skills:eval` actually runs them (it needs the
    `claude` CLI and real model calls, so it is deliberately **not** in the gate —
    run it when adding a skill or editing a model-invoked description).
    Two things learned writing the first set, both of which make a case measure