fix: replace prepare with prepack to fix git install

[monotykamary]

Problem

Installing from git via pi install git:github.com/mattleong/pi-code-previews fails with:

> pi-code-previews@0.1.25 prepare
> npm run build

> pi-code-previews@0.1.25 build
> tsdown

sh: tsdown: command not found

pi install runs npm install --omit=dev, which skips devDependencies. The prepare lifecycle script still fires and tries to run tsdown, but tsdown is a devDependency and isn't installed.

Fix

Replace the prepare script with prepack. The prepack lifecycle hook only runs before npm pack / npm publish, not during npm install. This means:

• npm publish still works — prepack builds dist/ before the tarball is created, so the published package contains the built output
• git install via pi no longer triggers a build, avoiding the tsdown: command not found error
• pi loads the extension via jiti from ./index.ts at runtime, so dist/ is never needed at git-install time

Why not other approaches?

Approach	Drawback
Move tsdown to dependencies	Semantically wrong (build tool ≠ runtime dep), bloats installs
Conditional prepare	Complex, noisy
Commit dist/ to repo	Messy git diffs, merge conflicts in generated files
npx tsdown in build script	Defeats --omit=dev, adds latency

Closes #7