Scope OpenCode submit_plan to planning agents

[backnotprop]

Summary

• add explicit OpenCode workflow modes: plan-agent default, manual commands-only, and all-agents legacy behavior
• make plan-agent the new default for 0.19.1: keep submit_plan available to OpenCode's built-in plan agent, allow any extra agents listed in planningAgents, and hide or deny it for build and other non-planning primary agents
• scope prompt injection, OpenCode permission mutation, and runtime execution checks around that workflow behavior
• keep manual features first-class across modes: /plannotator-last, /plannotator-annotate, /plannotator-review, and /plannotator-archive
• add OpenCode folder annotation support and fix folder draft collisions so drafts are isolated per folder instead of leaking between folders
• document the 0.19.1 migration, config snippets, troubleshooting, local sandbox flow, and manual test plan

Issues

Closes #422.

Refs #479. This PR implements the workflow split, the plan-agent default, and narrower default tool exposure, but /plannotator-last-plan and approval stay/stop semantics remain follow-up work.

Refs #118, #139, #159, #365, #295.

Testing

• bun test apps/opencode-plugin
• bun test apps/opencode-plugin/workflow.test.ts
• bun run build:opencode
• bun run build:pi
• bun run build:marketing