fix: run npm with Node's bin directory on PATH

[narcisonunez]

Fixes #5

Changes

• New getNpmEnv(npm) prepends dirname(npm) to PATH, applied to installPackage() and removePackage() (the latter passed no env at all).
• resolveNpm() now collects nvm versions as candidates instead of returning early on existsSync, and the single probe loop runs npm --version with that same env. This also fixes Homebrew: /opt/homebrew/bin/npm was failing the probe with exit 127 and being silently rejected, so a machine without nvm got "npm not found" outright. It also means the nvm result is actually validated rather than assumed.
• getNpmEnv() updates the existing PATH key case-insensitively. Windows spells it Path, and a plain spread of process.env keeps that casing, writing a separate PATH key would leave two, and Node keeps only the first, wiping the real search path.

Tests

3 new tests in package-manager.test.ts. The execSync mock models the actual failure: it rejects any command whose env lacks the Node bin directory, with env: node: No such file or directory. Each test asserts a branch that fails without this change — PATH prepended for install, the --version probe getting the same env, and a single PATH key when the env uses Path.

All tests are green

https://www.loom.com/share/45010ec5f4ce436fa5512e5c6c06000d

[vercel]

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[narcisonunez]

Good project. I would like to see the roadmap if possible. I want to be part of it :D