[agents-md] Dedup `upgrade`'s cwd check and trim verbose comments
`runUpgrade` now calls `requireNextProjectDir(cwd)` once at the top
and threads the resolved dir through `getInstalledNextVersion` and
the post-scaffold hook, replacing the duplicated
`require.resolve('next/package.json', { paths: [cwd] })` that was
happening in both places. `getInstalledNextVersion` collapses to
a two-line version read now that validation is a separate step.
Verbose JSDoc compressed across `lib/agents-md.ts`, `bin/upgrade-agents-md.ts`,
and `app-info-log.ts`. The docs/01-app/02-guides/ai-agents.mdx
`Existing projects` section collapses the monorepo note, the
post-upgrade note, and the upsert bullet list into a single
two-sentence lead — readers can grok the command without wading
through the fork logic.