test: add postinstall canary script to package.json

[pullfrog]

Changes to package.json made during the dependency installation behavior test run.

• Renamed package to test-pkg
• Added version field
• Replaced test script with a postinstall canary script
• Replaced dev dependencies with an empty dependencies object

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Note

Low Risk
Test-only package.json changes for install-hook verification; no production app or security-sensitive logic.

Overview
Reconfigures package.json as a minimal test-pkg fixture to verify postinstall runs during dependency installation.

The package is renamed and given 1.0.0, prior private, type, and vitest setup are removed, and a postinstall script writes CANARY_MARKER to /tmp/postinstall-canary.txt. dependencies is left empty so installs exercise script hooks without pulling packages.

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