fix(opencode): preserve planning handoff

[franktronics]

Summary

• Prevent the OpenCode plan agent and configured planning agents from receiving the Proceed with implementation handoff after plan approval.

• Keep the implementation handoff unchanged for build and other non-planning agents.

• Emit a visible status message when approval targets a planning agent:

  Plan approved. Plan mode remains active; no implementation has been requested.

Related issues

Regression from #648: the agent picker correctly removed the implementation directive for No switch, but selecting plan still injected Proceed with implementation.

#648 also addressed #159, which requested approval without automatically starting implementation. This change preserves that behavior when the selected target is a planning agent.

Changes

File	Change
apps/opencode-plugin/workflow.ts	Adds a planning-target predicate for implementation handoffs.
apps/opencode-plugin/index.ts	Sends the implementation prompt only to non-planning agents; planning agents receive a non-executing status message instead.
apps/opencode-plugin/workflow.test.ts	Covers build, plan, custom planning agents, and regular non-planning agents.

Test plan

• bun test apps/opencode-plugin
• bun run build:opencode
• Manual: selecting plan keeps plan mode active and displays the status message.
• Manual: selecting build still sends Proceed with implementation.