test: add package.json with postinstall canary

[pullfrog]

Adds a package.json written during the dependency installation canary test. The postinstall script writes a marker file to /tmp/postinstall-canary.txt to verify whether npm lifecycle scripts run during start_dependency_installation.

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Note

Low Risk
Test-only artifact with a local postinstall side effect; no production app or auth/data paths touched.

Overview
Introduces a minimal package.json for the dependency-installation canary test, replacing the prior Vitest-oriented package metadata.

The postinstall script writes CANARY_MARKER to /tmp/postinstall-canary.txt so the test harness can confirm whether start_dependency_installation runs npm lifecycle scripts. Package name/version are set to test-pkg / 1.0.0; dependencies is empty.

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