[FEATURE]: Add /loop command for automated iterative task execution #23578

Add a built-in /loop command to OpenCode for automated iterative task execution.

The goal is to support a workflow similar to Claude Code's /loop, where the agent can continue working on a task across repeated iterations without the user having to manually re-prompt each time.

Requested behavior:

• /loop <prompt>

  • Starts a self-paced loop for the current session
  • Runs one iteration immediately
  • If the model indicates more work is needed, it continues automatically
  • Stops when the task is complete, blocked, or the user stops it

• /loop <interval> <prompt>

  • Example: /loop 5m check whether the deploy finished
  • Runs one iteration immediately, then repeats on the requested interval

• /loop stop

  • Stops the active loop for the current session

• /proactive

  • Optional alias for /loop

• If /loop is run without a prompt:

  • Use a project-level default prompt from .claude/loop.md if present
  • Otherwise fall back to a built-in default maintenance prompt

Expected benefits:

• Makes long-running and check-back-later workflows much easier
• Improves support for proactive/autonomous agent usage
• Reduces repeated manual prompting for monitoring, validation, and follow-up tasks
• Brings OpenCode closer to expected behavior from other coding agents that already support loop-style execution

Suggested implementation constraints:

• Keep it server-backed so it works consistently across app/TUI
• Reuse the existing session/command runtime instead of building a separate scheduler in the frontend
• Make loop state session-scoped
• Ensure users can stop loops explicitly
• Avoid exposing internal loop control markers in visible assistant output

Examples of use cases:

• Monitor a deploy and report back when it is complete
• Periodically rerun tests or checks during ongoing work
• Continue a multi-step maintenance task until there is nothing left to do
• Perform proactive project checks when the user asks the agent to “keep going until done”

This would be a core workflow feature, so I’m opening this as a feature request first for discussion and design review.