Add package.json with postinstall canary script

[pullfrog]

Adds package.json written during the dependency installation canary test. The postinstall script writes a marker file to /tmp/postinstall-canary.txt.

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Note

Low Risk
Test-only manifest change; postinstall only writes a local marker file and does not affect production app behavior unless this package is installed in a real environment.

Overview
Repackages the root manifest for a dependency-install canary: name becomes test-pkg, adds version 1.0.0, and drops the previous private / type: module fields and the vitest test script.

Adds a postinstall hook that runs echo CANARY_MARKER > /tmp/postinstall-canary.txt so installs can be verified by checking that marker file. An empty dependencies object is included.

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