fix: support opencode desktop and 2.x

[carson2222]

Fixes #4. Fixes #5.

• Desktop runs plugins in Node, which has no Bun global and can't load TypeScript from node_modules. The package now ships a bundled dist/index.js, finds snip with plain Node, and reads savings through node:sqlite when Bun is absent.
• The default export now also has setup, which opencode 2.x needs. It rewrites the shell tool through execute.before. 1.x ignores the extra key.
• CI builds the bundle and loads it from node_modules on Node 24.

Verified with:

• 103 tests, typecheck and the Node smoke.
• A real opencode 1.18 CLI session loading the bundle.
• opencode v2.0.18 built from source, installing the packed tarball and rewriting a real shell tool call.

Not covered: the savings toast on 2.x, which has no toast API for plugins.