[FEATURE]: Add 'opencode agents' command for background session management

[alanZee]

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Describe the enhancement you want to request

Feature Description

Add an opencode agents command that provides an interactive TUI interface for managing background agent sessions, similar to Claude Code's claude agents command.

Motivation

Claude Code has a claude agents command that opens an interactive TUI view for managing background coding sessions. This allows users to:

• View all running background sessions at a glance
• Switch between sessions easily
• Monitor session status and progress
• Dispatch new sessions with specific settings (model, effort level, permissions)
  Currently, OpenCode manages background sessions through CLI commands (opencode session list, opencode -s <id>, opencode -c), which lacks the convenience of a unified interactive interface.

Proposed Solution

Add a new opencode agents command that:

1. Opens an interactive TUI (similar to the main OpenCode TUI)
2. Lists all background sessions with their status, model, and working directory
3. Allows selecting a session to attach/resume
4. Provides options to start new sessions with configurable settings
5. Shows session statistics (token usage, cost, duration)
   Example usage:

# Open the agents management TUI
opencode agents
# Filter by working directory
opencode agents --cwd /path/to/project
# Set default model for new sessions from the TUI
opencode agents --model anthropic/claude-sonnet-4
# Set default effort level
opencode agents --effort high
Alternatives Considered
1. 
Enhance existing opencode session list: Could add interactive selection, but doesn't provide the full management experience
2. 
Add flags to opencode command: Could add --list-sessions flag, but doesn't provide a unified view
3. 
Create a separate opencode-tui tool: Could be a plugin, but core integration would be more seamless
Additional Context
This feature would align OpenCode with Claude Code's session management UX, making it easier for users to manage multiple concurrent coding tasks across different projects.
**Label:** `enhancement`

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• [FEATURE]: Pinned session tabs in the TUI sidebar #24451: Proposes a SESSIONS panel in the TUI sidebar for parallel session orchestration — pinning/unpinning sessions, live status indicators (spinner/idle), and quick switching between sessions. Very closely related to the session management UX this issue describes.
• feat(tui): Chrome-style tab system for multi-session workflows #12548: Proposes a Chrome-style tab system for managing multiple simultaneous sessions in the TUI, with tab navigation keybinds and per-tab status indicators.

Please review these issues to see if your request is already covered or if your proposal meaningfully differs from them.