Remove dev-only platform binaries from optionalDependencies

[gh0stonio]

Summary

Fixes #222, removes @oven/bun-* and @rollup/rollup-* from optionalDependencies entirely, and removes the redundant postinstall script.

Builds on the investigation from #451, thanks @ochafik! This is a simpler alternative to #516.

Context

At Datadog we're building on MCP Apps and this optionalDependencies bloat (~400MB of platform binaries) is blocking our CI pipelines, which motivated picking this up.

Why #451's approach doesn't work

#451 moved the platform binaries from optionalDependencies to devDependencies. These packages declare os/cpu constraints in their own package.json, npm silently skips mismatches in optionalDependencies but fails hard in devDependencies (e.g. npm ci on macOS ARM errors on @oven/bun-linux-x64).

Approach

Instead of moving deps around, just remove them — they're not needed:

• @rollup/rollup-*: rollup already declares its own platform-specific optionalDependencies, npm resolves them transitively, the top-level entries are redundant
• @oven/bun-*: setup-bun.mjs (run by prepare) already gracefully handles missing binaries with a "install bun separately" fallback. Contributors who need bun can install it globally via bun.sh
• Removed postinstall: redundant with prepare (which already runs setup-bun.mjs), and the script isn't even shipped in the published package ("files": ["dist"])

Test plan

• npm ci succeeds
• npm run build succeeds
• npm pack — published package.json contains no optionalDependencies

[gh0stonio]

Alternative to #516 which takes a prepack/postpack approach to strip the entries at publish time instead of removing them entirely.

[gh0stonio]

Closing in favor of #516. This approach (removing optionalDependencies entirely) breaks contributor DX since bun setup relies on them being installed before prepare runs.