Feature: Inline model/agent override in task runner + settings UI

[jellydn]

What

Add inline agent/model selector when running tasks from ralph-tui, plus settings UI for persistent agent configuration.

Why

Current limitation:

• CLI only: ralph-tui run --agent opencode --model X
• opencode's default_agent: "plan" forces plan review on every task
• Can't quickly switch models from TUI for different tasks

Desired workflow:

Select task → Run → Inline picker shows:
  Agent: [opencode ▼]  Model: [claude-3.5-sonnet ▼]  Mode: [build ▼]

How

Part 1: Inline Override (Task Runner)

When executing a bead/task, show inline options:

┌─────────────────────────────────────┐
│ 📋 Task: Implement feature X        │
├─────────────────────────────────────┤
│ Agent: [opencode ▼]                 │
│ Model: [claude-3.5-sonnet ▼]        │  ← NEW
│ Mode:  [build ▼]                    │  ← NEW: build vs plan
│                                     │
│       [Cancel]  [Run →]             │
└─────────────────────────────────────┘

• Inline changes = one-time override (don't persist)
• Mode build = direct execution (bypass plan review)
• Mode plan = plan mode first (requires approval)

Part 2: Settings UI

Add agent configuration to settings screen:

⚙️  Settings
┌─────────────────────────────────────┐
│ Default Agent: [opencode ▼]         │
│ Default Model: [claude-3.5-sonnet ▼]│
│ Default Mode:  [build ▼]            │
└─────────────────────────────────────┘

Supported models:

• anthropic/claude-3-5-sonnet
• claude-opus-4
• openai/gpt-4o
• openai/gpt-4o-mini
• openai/o1

Part 3: Config Schema

{
  "agents": {
    "default": "opencode",
    "opencode": {
      "model": "anthropic/claude-3-5-sonnet",
      "mode": "build"
    }
  }
}

Files to Modify

• src/config/agent-config.ts - Config schema
• src/ui/screens/settings.tsx - Settings UI
• src/ui/components/task-runner.tsx - Inline selectors
• src/agents/opencode.ts - Pass model/mode to opencode

Acceptance Criteria

• Settings UI has agent/model/mode configuration
• Task runner shows inline selectors before execution
• Inline changes are one-time (settings unchanged)
• Settings changes persist across sessions
• Mode build bypasses plan review
• Mode plan respects plan flow
• Documentation updated

User Flows

One-off model change:

1. Select task → Run
2. Change model to "gpt-4o" in picker
3. Click Run → executes with gpt-4o
4. Next task uses default model

Persistent preference:

1. Settings → change default to "claude-opus-4"
2. All tasks use this model (unless overridden inline)

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Labels: feature, enhancement, ux, settings-ui, opencode