fix(skill): bypass host packageManager pin in Phase 0 build (#315)

[tirth8205]

Summary

Closes #315.

When /understand runs from a host project whose package.json pins "packageManager": "yarn@x" or "npm@x", corepack reads the shell CWD's pin (not pnpm's -C / --dir target) and refuses to launch pnpm — failing the Phase 0 plugin build before any analysis can begin.

• SKILL.md Phase 0 — wrap each pnpm invocation in (cd "$PLUGIN_ROOT" && COREPACK_ENABLE_PROJECT_SPEC=0 pnpm …). The subshell cd makes corepack's CWD-based lookup hit the plugin's own packageManager pin; the env var disables corepack project-spec enforcement entirely as a backstop. Added a maintainer note inline so future cleanups don't strip the guards.
• understand-anything-plugin/package.json — add the same "packageManager": "pnpm@10.6.2+sha512.…" value used by the root package.json, so corepack inside the plugin dir always picks pnpm (e.g., when running from the marketplace cache copy where the root package.json isn't a parent).
• pnpm-lock.yaml — regenerate. The shipped lockfile was missing the graphology / graphology-communities-louvain importer entries (added in earlier PRs to the plugin's package.json) plus a stale vite pin in the dashboard workspace, so pnpm install --frozen-lockfile was already failing with ERR_PNPM_OUTDATED_LOCKFILE before the corepack issue could even surface — the issue's second papercut.

Test plan

• pnpm install --frozen-lockfile from understand-anything-plugin/ succeeds (was failing before with ERR_PNPM_OUTDATED_LOCKFILE).
• pnpm --filter @understand-anything/core build from understand-anything-plugin/ succeeds.
• Manual repro: run /understand from a project whose package.json pins "packageManager": "yarn@4.x" and confirm Phase 0 build no longer fails with This project is configured to use yarn.

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