feat: support pnpm / yarn / bun in purus new and purus init

[otnc]

Summary

purus new hardcoded npm init / npm install. This PR adds pkg/lib/pm.js, which detects the package manager instead of assuming one — nothing is hardcoded to a specific manager:

1. npm_config_user_agent — set by the launcher (npx, pnpm dlx, yarn dlx, bunx, and <pm> run scripts), so pnpm dlx purus new my-app naturally uses pnpm
2. Lockfile in the target directory (package-lock.json, pnpm-lock.yaml, yarn.lock, bun.lock / bun.lockb) — relevant for purus init in existing projects
3. npm as the final fallback

What the detected manager drives

• package.json init (npm init [-y] / pnpm init / yarn init [-y])
• devDependency install (npm install --save-dev / pnpm add -D / yarn add --dev / bun add --dev)
• the generated README (install command and <pm> run ... script table)
• post-create hints (bun install etc.)
• purus init: suggests <pm> init when no package.json exists

For bun, whose bun init scaffolds extra files (index.ts, tsconfig), a minimal package.json is written directly instead.

Tests

Manually verified on Windows:

• user-agent detection for pnpm / yarn / bun / npm / unknown (falls back to npm)
• purus new demo -y with npm: npm init -y + auto-install ran, scripts added
• purus new demo with bun user agent: minimal package.json written, README and hints use bun

Fixes #3