Add package.json for dependency install canary test

[pullfrog]

Adds a package.json written during the dependency installation behavior test. The file defines a postinstall script that writes a canary marker to /tmp/postinstall-canary.txt, used to verify whether postinstall hooks run during start_dependency_installation / await_dependency_installation.

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Note

Low Risk
Test-only manifest change with no production runtime impact; the postinstall writes a local temp file during install tests.

Overview
Replaces the prior Vitest-oriented package.json with a minimal test-pkg manifest used only for dependency-install behavior checks.

The new file adds a postinstall script that writes CANARY_MARKER to /tmp/postinstall-canary.txt, so tests around start_dependency_installation / await_dependency_installation can tell whether install hooks actually ran. private, type: module, and the vitest script are removed; version and an empty dependencies object are added.

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