feat(opencode): Reflect native Plan and Build selections

[mwolson]

What Changed

When OpenCode switches its root session between the Plan and Build agents during a turn T3 started, the thread's interaction mode in T3 Code follows, so the composer shows Plan or Default to match. Only live selections from that thread's own root session count, and only ones OpenCode made after the turn started. Child sessions, replayed events, quarantined or stopped work, and background activity cannot change the mode.

A user's own change to the mode, runtime mode, model or provider after the turn started always wins, and a thread an agent created in Plan mode is never switched back automatically. Each reflection is applied once, even if it is retried.

Why

Switching agents inside OpenCode left T3 showing the old mode, so the next message was sent with a mode the agent had already left. Following only the turn T3 admitted, and re-checking the thread under its lock before applying, keeps stale or foreign selections from overriding the user.

UI Changes

The composer's Plan and Default toggle can change on its own to follow OpenCode. No screenshots are attached.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

[mwolson]

Replaced by Julius's upstream OpenCode 2 stack (pingdotgg#14239 oc2-1-runtime-probe onward), now carried on the fork as #45 oc2-4b-adapter-usage through #51 opencode2-continuation-ownership. Closing; the branch is kept.