feat: add root package.json so dsh-plugin marketplace scanners can discover the plugin - #7

Why

The DSH plugin marketplaces (e.g. YELEBAI/dsh-plugin-marketplace, w2112515/dsh-plugin-marketplace) discover plugins by auto-scanning the dsh-plugin GitHub topic and reading the repository root package.json. dsh-mneme's manifest currently lives only at dsh-mneme/package.json, so the scanners fail with package-json-missing and dsh-mneme cannot be listed/validated.

This adds a root package.json that mirrors the real manifest and points every DSH entry at the existing subdirectory files:

• dsh.bundle.patch → ./dsh-mneme/cordis.patch.yml (already exists, inserts loader entry @modusensus/dsh-mneme)
• dsh.client (web platform) + exports["./client"] → ./dsh-mneme/lib/client.js (already exists)
• main/exports["."] → ./dsh-mneme/lib/index.js
• dsh.marketplace profile/install hints so scanners can classify the install

No existing layout is touched, and the npm package (@modusensus/dsh-mneme) is still published from dsh-mneme/.

Effect

• YELEBAI, w2112515 and similar topic-scanning marketplaces will now be able to validate and list dsh-mneme automatically on their next scan.
• UntR and AwesomeHou marketplaces (live topic sync) continue to list it.