chore(opencode): move V2 adapter to the stable plugin API #1196

The OpenCode 2 adapter added in #1194 currently pins a matching next release of @opencode-ai/plugin and excludes its five-package V2 dependency family from Bun's seven-day release-age gate.

The current stable package, @opencode-ai/plugin@1.18.13, exposes Plugin.define through @opencode-ai/plugin/v2/promise, but that version's plugin context does not include the session or tool domains required for Plannotator's context transformation and submit_plan registration. It therefore cannot replace the pinned nightly yet.

When those APIs reach a stable release:

• Move the OpenCode 2 adapter to the stable plugin export.
• Revisit the V1 build boundary so dist/index.js does not bake the nightly /v1 helper into the published bundle when a stable equivalent is available.
• Remove the @opencode-ai/ai, @opencode-ai/client, @opencode-ai/plugin, @opencode-ai/protocol, and @opencode-ai/schema release-age exclusions from bunfig.toml.
• Update the installed-package CI smoke to the matching stable opencode2 release.
• Re-run the OpenCode 1 package-boundary coverage and the real OpenCode 2 packed-package smoke.

Follow-up to #904 and #1194.