add package.json with postinstall canary script

[pullfrog]

Adds a package.json written during the dependency installation behavior test (PR #1022). The postinstall script writes a canary file to /tmp/postinstall-canary.txt; the canary was not produced because npm install failed with an unknown error.

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Note

Low Risk
Test-only manifest change; the postinstall script is intentional for install verification and does not affect application runtime in normal use.

Overview
Replaces the repo’s minimal Vitest-oriented package.json with a test-pkg fixture used for dependency-install behavior tests.

The manifest drops private, type: module, and the test script, adds version 1.0.0, an empty dependencies, and a postinstall script that writes CANARY_MARKER to /tmp/postinstall-canary.txt so installs can be verified when npm install succeeds.

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