Add plugin entry: opencode2

[Flame119052]

What the plugin does

Adds OpenCode 2 as an ACP provider in BB, translates recognized BB /skill
invocations into OpenCode 2 native skill-tool loads, and shows one shared
OpenCode Go allowance across regular OpenCode and OpenCode 2. The usage UI
uses the rolling five-hour window in the compact sidebar and also renders
weekly/monthly details in Settings, desktop, remote, and mobile clients. It
also shows Cursor plan usage with an independent toggle and a compact,
responsive four-provider layout.

Source release

• Repository: https://github.com/Flame119052/bb-plugin-opencode2.git
• Release: v1.0.1
• Marketplace range: ^1.0.0

Plugin checks

• BB SDK types check passed against SDK 0.4.15.
• TypeScript typecheck passed.
• Ten usage, Cursor, mobile, settings, and skill-routing tests passed.
• BB server, app, and host bundles built successfully.
• Live BB smoke test discovered 29 OpenCode 2 models and loaded a configured
  skill through the native skill tool without an approval interaction.

Marketplace checks

• Entry schema/build passed locally.
• Vendored SVG icon is 1,760 bytes and content-hashed.
• Full marketplace liveness/source verification passed against the public
  immutable v1.0.1 tag selected by the existing ^1.0.0 range.

Permissions and external services

The plugin is full-trust provider code and launches the locally installed
opencode2 CLI. Its optional secret setting is used server-side to query the
OpenCode Go usage endpoint; the key is never returned to the browser. The
frontend stores only the last successful usage snapshot for failure recovery.

The usage UI is derived from Mateo Cerquetella's MIT-licensed Usage Tracker,
and the ACP bridge is adapted from BB's MIT-licensed provider-acp plugin. Both
are credited in the included license and third-party notices.