ci: pin validator deps, check SKILL.md frontmatter, run script-package tests

[deanrie]

Summary

One PR for the repo-level CI and validator work from #531, replacing #607, #599 and #598 (closed in favour of this one; their commits are carried here unchanged, one commit each, reviewable separately). Together they close pieces 2, 3b and 4 of the shape in #531; #582 (akushonkamen) covers pieces 1 and 2a and touches validate-plugins.yml / validate-plugins.mjs too, so whichever lands second needs a small rebase.

1. ci: pin the validator's ajv dependencies in a root package.json and install with npm ci (was ci: pin the validator's ajv dependencies in a root package.json and install with npm ci #607). The job installed ajv and ajv-formats unpinned on every run. Root package.json (private, devDependencies only, validate script), package-lock.json (7 packages), npm ci with the npm cache, and the paths: trigger widened to scripts/**, the package files and the workflow.
2. validate: check SKILL.md frontmatter (name, description, folder match, length) (was validate: check SKILL.md frontmatter (name, description, folder match, length) #599). The validator never read a SKILL.md. Errors: no frontmatter, missing name, missing description. Warnings for now: name not kebab-case or not equal to the folder, description over 1024 characters. On current main that is 0 errors and 5 warnings (cursor-sdk description, two pstack and two third_party names), all with open fix PRs (chore: keep the cursor-sdk skill description under the 1024-char limit; label agent-compatibility changelog 1.0.0 #530, fix(pstack): make skill names match their directories (poteto-mode, make-bot-ui) #596, fix(x, x-money): make skill names match their directories #600); once those land the two warnings can become errors in a two-line follow-up.
3. ci: run bun typecheck and tests for the two script packages on PRs (was ci: run bun typecheck and tests for the two script packages on PRs #598). orchestrate/skills/orchestrate/scripts (209 tests) and pstack/skills/poteto-mode/scripts (63 tests) have test suites nothing ran. New script-packages.yml: matrix over the two directories, bun install --frozen-lockfile, bun run typecheck, bun run test, fail-fast: false, paths: scoped to the two packages and the workflow.

Verification

• rm -rf node_modules && npm ci && npm run validate on this branch: All plugins validated successfully. with the 5 expected warnings.
• Both workflow files parse as YAML.
• Script packages checked locally with bun 1.4.3: orchestrate tsc ok, 209 pass / 0 fail; poteto-mode tsc ok, 63 pass / 0 fail.
• As a fork PR the workflow runs will sit in "approval required" until a maintainer approves them.

Refs #531.