♻️ Replace verify skill with an npm verify script

A skill duplicated AGENTS.md policy and only worked in Agent-Skills harnesses.
A bare top-level script is the repo-native affordance: every harness (and CI,
and a human) reads package.json#scripts as the source of truth. Add
`npm run verify` = `npm run check` + `test:unit` (tsc runs once, since check
already type-checks) as the one-shot whole-project correctness gate.

Delete .agents/skills/verify/SKILL.md. Document verify in README (Development +
a Tooling-decisions bullet + bare-command note in the prefix list), CONTRIBUTING
(feedback-tier table, "Before pushing", the bare-command paragraph, a "why these
splits" bullet), and AGENTS.md (test = fast iterating gate, verify = definition
of done; skill pointer removed).

package.json

@@ -50,6 +50,7 @@
         "test": "npm run check:tsc && node --test --strip-types \"src/**/*.test.ts\"",
         "test:ci": "c8 --reporter=text --reporter=lcov --reporter=html node --test --strip-types \"src/**/*.test.ts\"",
         "test:unit": "node --test --strip-types \"src/**/*.test.ts\"",
+        "verify": "npm run check && npm run test:unit",
         "watch": "npm run watch:test",
         "watch:test": "node --test --watch --strip-types \"src/**/*.test.ts\"",
         "publish:attw": "attw . --pack --profile esm-only",