feat(plugin): add goal-loop example plugin

[charleneleong-ai]

Issue for this PR

Closes #27167

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Example plugin showing how to build /goal and /loop using the plugin SDK. No core changes, no SQLite, no auto-continuation. Just tools + slash commands.

Turn control

The goal tool supports optional turn limits:

/goal fix the broken tests max 20 turns
/goal write documentation one task per turn
/goal refactor auth module max 10 turns one task per turn

• maxTurns - agent stops after N turns and reports progress
• oneTaskPerTurn - agent does exactly one step per turn, then waits

These are enforced by the slash command prompt, not hard-coded in the plugin. The agent checks goal({ action: "status" }) after each turn and stops when limits are reached.

How it is different from other PRs

#45379 (thdxr) - core implementation with auto-continuation via event worker. Ours is a pure plugin SDK example, no core changes. Different scope.

#28610 (closed) - auto-continuation via idle events + client.prompt. Ours does not do that. User drives with slash commands.

#32743 (draft) - SQLite + core runtime changes. Ours is just file-based state in .opencode/harness/.

@bojackduy/opencode-loopd - full production plugin with subagents, TUI dashboard, scheduled goals, workspace serialization. Way more complex (1000+ lines vs our ~90). Ours is a minimal example for learning the plugin SDK. Theirs is a production goal engine.

	Our plugin	opencode-loopd
Lines	~90	1000+
Auto-continuation	No	Yes (engine-driven)
Subagents	No	Yes (child sessions)
TUI dashboard	No	Yes (modal, vim-style)
Scheduled goals	No	Yes
State	Simple JSON	Complex + artifacts
License	MIT	AGPL-3.0
Purpose	Example plugin	Production tool

Free models work because the slash command injects a prompt telling the agent to call the tool. No tool-calling needed from the model side.

How did you verify your code works?

• Plugin loads without errors on openai/gpt-4o
• goal set/status/clear works via tool calls
• goal with maxTurns stores limit and shows in status
• goal with oneTaskPerTurn stores flag and shows in status
• loop add/list/remove works via tool calls
• /goal slash command triggers the tool with turn control

Tests

Added packages/plugin/src/examples/goal-loop.test.ts (bun:test, 10 tests):

• goal: set + clear round-trip, status with active goal, status when no goal, set without condition
• goal: set with maxTurns, set with oneTaskPerTurn
• loop: add + list round-trip, remove loop, list when no loops, add without prompt

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR