fix: report pi scrub package version

[rynfar]

Change

The published 0.2.1 package reports plugin version 0.2.0. Read the version from the shipped package.json so Release Please's package version and the runtime plugin metadata stay aligned. Add a regression assertion; prompt scrubbing behavior is unchanged.

Validation

• Baseline with the new assertion: 2 pass, 1 fail (0.2.0 reported versus package 0.2.1).
• npm ci, npm test (14 pass), npm run build, Node import of built plugin, and npm pack dry run passed.
• Real Pi 0.72.1 → isolated Meridian 1.76.3 → Claude Agent SDK / Haiku 4.5 returned successfully. The plugin loaded as version 0.2.1, and the pre-hook Pi identity/docs were absent at the SDK while generic identity remained. Local evidence artifact: pi-scrub-live-TmGUw3.
• The Nix package layout is being corrected in Meridian PR #1144 to preserve the metadata file imported at runtime.