feat: add native OpenCode V2 plugin support

[pascalandr]

Summary

Adds native OpenCode V2 compatibility while preserving the OpenCode V1 root entrypoint.

• tracks @opencode-ai/plugin through the published beta dist-tag
• exposes a pure Plugin.define({ id, setup }) V2 ./server export
• starts and cleans up the existing Bun Claude proxy
• adds the claude-code provider and six Claude models with effort variants
• associates the provider with its Claude CLI authentication integration
• routes V2 model requests through the proxy while preserving effort, session, and directory headers
• keeps V1 code on a stable-major dependency alias because the beta package no longer exports V1 types

Configuration

{
  "$schema": "https://opencode.ai/config.json",
  "plugins": ["@openchamber/opencode-claude"],
  "providers": {
    "claude-code": { "name": "Claude Code" }
  }
}

The package root remains the V1 entrypoint. OpenCode V2 package installation selects ./server; that export intentionally has no legacy server property, preventing the V2 loader from classifying it as a V1 plugin.

Verification

• bun run build
• bunx tsc -p tsconfig.json --noEmit
• focused V2 test covering module shape, catalog, provider/integration association, request routing, headers, and cleanup
• npm pack --dry-run
• packed root and ./server imports
• real V1 catalog smoke
• isolated V2 setup/proxy smoke

Limitations

• The complete smoke suite still reaches the existing Windows extensionless-executable fixture failure at test/smoke.ts:236.
• Interactive Claude login and authenticated inference require account interaction and were not automated.
• The V2 API remains beta, so updates to the beta dist-tag require the same build and runtime validation.