[FEATURE]: Config-driven agent system — no hardcoded agents, all behavior defined in global config #12498

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Currently, all 7 built-in agents are a mix of config and hardcoded logic. general, explore, compaction, title, and summary are fully defined via config (permissions, prompts, mode). But build and plan have significant behavior hardcoded in prompt.ts — the insertReminders function string-matches agent.name === "plan" / agent.name === "build" to inject system-reminder blocks, plan file management, mode transition prompts, and the 5-phase planning workflow.

This means the agent system isn't truly modular. You can create custom agents, but you can't fully replace or extend the two most important ones.

Proposal: Fully config-driven agent system

Refactor so that zero agents are hardcoded. The agent runtime should be agnostic — it doesn't know or care about "build" or "plan" by name. Instead:

1. All agent behavior defined in global config — ship the current build/plan/general/explore/compaction/title/summary as default config entries (in ~/.config/opencode/ or bundled defaults). The runtime loads and executes them generically.

2. Mode transitions as config — instead of hardcoding "when switching from plan to build, inject this reminder", define transition prompts as part of the agent config. Something like:

   {
     "plan": {
       "on_exit": "{file:./prompts/build-switch.txt}",
       "on_enter": "{file:./prompts/plan-reminder.txt}"
     }
   }

3. Dynamic injection via plugin system — use experimental.chat.messages.transform for things like plan file path injection, rather than hardcoding it in prompt.ts. Ship this as a built-in plugin that users can override or extend.

4. Template variables for dynamic content — plan file paths, session IDs, etc. could use variables like {plan_file}, {session_id} in prompt templates instead of being assembled in TypeScript.

What this enables

• Full control — users can customize every aspect of every agent without forking
• Custom workflows — build your own planning flow, review flow, or multi-phase agent choreography entirely through config
• More programmatic use — agent behavior becomes data, not code. Easier to generate, compose, and share agent configs
• Cleaner codebase — the runtime becomes a generic agent executor. Agent-specific logic lives in config where it belongs
• Default behavior unchanged — new installs ship with the same defaults, just loaded from config instead of hardcoded

Happy to take this on if the team is interested.