feat: Subagent output panel with Ctrl+X navigation

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Feature

Implement a subagent output panel in oclite, similar to the opencode TUI's task panel:

• Buffer output from the last 5 subagent runs
• Navigate between subagent views with Ctrl+X + ←/→ arrows
• Ctrl+X + ↑ returns to main screen
• Show subagent name, status, and elapsed time in panel header

UX Design

Main view:  project-manager ❯ █
            [Ctrl+X →] to view subagent: @developer (running, 12s)

Panel view: ┌ @developer: Implement feature ──────────────┐
            │ Reading file...                              │
            │ ✓ bash  git status                           │
            │ Writing implementation...                    │
            │                                              │
            │ [Ctrl+X ←/→] prev/next  [Ctrl+X ↑] back     │
            └──────────────────────────────────────────────┘

Implementation Approach

• Research the opencode TUI's panel implementation first
• Buffer subagent session output as streaming chunks arrive
• Ring buffer of 5 most recent subagents
• Full-screen panel takeover on navigation (not a split pane)
• Subscribe to subagent session events via Bus

Related

• [Architecture] oclite: Live Block rendering with log-update for tools, tasks, and todos #82 (Live Block — shows subagent status inline)
• [UX] oclite: Live subagent status panel #68 (live subagent status — enabled by this)

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Research Findings: OpenCode TUI Architecture

Session Model

• Each subagent creates a separate Session with parentID linking to parent
• Session.children(parentID) retrieves all child sessions
• Messages stored independently per session via MessageV2.stream(sessionID)

Keyboard Navigation

• Ctrl+X → Right Arrow — next child session (session_child_cycle)
• Ctrl+X → Left Arrow — previous child session (session_child_cycle_reverse)
• Ctrl+X → Up Arrow — back to parent session (session_parent)
• Leader key default: ctrl+x

Key Implementation Files

• packages/opencode/src/session/index.ts — Session.children(parentID), Info.parentID
• packages/opencode/src/tool/task.ts line 174-204 — creates child session with parentID
• packages/opencode/src/cli/cmd/tui/routes/session/header.tsx — subagent header with Prev/Next/Parent buttons
• packages/opencode/src/cli/cmd/tui/routes/session/index.tsx line 310-327 — moveChild(direction) cycling logic
• packages/opencode/src/config/config.ts line 789-791 — keybinding definitions

Proposed oclite Implementation

No custom buffering needed — sessions already store everything. Implementation plan:

1. Track current view state: mainSessionID vs viewingChildID
2. On task tool_start: Record child session IDs as they appear
3. Ctrl+X + →/←: Cycle through Session.children(currentSessionID), limited to last 5
4. Ctrl+X + ↑: Return to main session view
5. Panel render: Clear screen, show child session header + streamed messages, show navigation hints at bottom
6. Live updates: Subscribe to child session's PartUpdated events while viewing

Estimated complexity: ~200-300 lines across 2 files (panel renderer + key handler integration)

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Implemented in commit f325255 — feat(cli): add subagent output panel with Ctrl+X navigation