Support OpenCode native goals in T3 Code's goal UI

[zeromask1337]

OpenCode 2.2.5 ships a native /goal plugin. T3 Code mirrors native goals for Codex and Claude, but OpenCode goal state never reaches the shared goal UI, and goal dispatch excludes the opencode driver.

Proposed scope

• Recognize opencode as a goal-capable driver in orchestration and turn start.
• Read opencode.goal over the plugin RPC and subscribe to rpc.opencode.goal.changed.
• Map native active, paused, and complete state into the provider-thread goal.
• Keep one T3 run open across the goal plugin's continuation executions.
• Settle show, pause, and clear commands locally because they start no native execution.
• Attach the goal above the composer, matching the Codex/Claude goal presentation.

Out of scope

• Changing OpenCode's goal plugin or its reviewer.
• Adding a T3-side goal loop for providers without native goals.

Verification

• Focused OpenCode adapter replay tests for set, show, pause, and goal continuation.
• Manual run against OpenCode 2.2.5: /goal creates a proof file, the goal bar shows Pursuing goal, then Goal complete.
• Before/after screenshots will be attached to the PR.