test: write package.json for dependency install canary test

[pullfrog]

Adds a package.json written as part of the dependency installation canary test (PR #688). This replaces the existing package.json with a test fixture used to verify postinstall script behavior.

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Note

Low Risk
Test-only package.json change for a canary; no production app logic, though the postinstall writes to /tmp when npm install runs in that environment.

Overview
Replaces the existing package.json (Vitest-based test-repo setup) with a minimal canary fixture for the dependency-installation test from PR #688.

The new manifest names the package test-pkg, adds version and an empty dependencies object, and defines a postinstall script that writes CANARY_MARKER to /tmp/postinstall-canary.txt so installs can be verified. The prior private, type: module, and test script are removed.

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