Add ACP tool call reporting and setSessionMode for Goose UI

[codelogiq]

Phase 12 of Goose added plan mode and subagent UI but the ACP protocol is missing two capabilities:

1. setSessionMode — mid-session plan/build mode switching (OpenCode TUI uses Tab internally)
2. Tool call reporting — visibility into tool execution (needed for subagent panel/badge/timeline)

Both are listed as 'not yet implemented' in packages/opencode/src/acp/README.md.

Goose UI components are gated with GATED comments — search these files:

• OrganicRail.tsx (PlanModeToggle, SubagentPanel, Network icon)
• AgentTitleBar.tsx (SubagentBadge)
• ConversationView.tsx (PlanCard, SubagentInlineIndicator)
• AgentsCategory.tsx (Default Plan Mode setting)

All backend wiring is in place — just needs ACP protocol support.