[P1][OpenCode] Replace hosted ContextGatheringAgent chat with OpenCode sessions

[isayahc]

Objective

Make OpenCode the hosted browser chat agent instead of ContextGatheringAgent and the current Forma server-side conversation flow.

Current locations

• apps/api/context_gathering_api.py:context_gathering_agent() constructs the current agent.
• apps/api/context_gathering_api.py:gather_project_context_endpoint() owns the current conversation/build handoff.
• apps/web/app/forma-workspace.tsx submits /projects/{project_id}/context/messages, /generate, and /iterate requests.
• apps/web/app/forma-workspace/home-chat-view.tsx renders context suggestions, build-now actions, and current progress.

Required behavior

• OpenCode owns the conversation, project authoring, and revision intent.
• Forma owns typed Hardware IR compilation, deterministic validation, rendering, persistence, and revision ownership.
• Use session/message/event APIs backed by durable Vercel records.
• Do not silently fall back to ContextGatheringAgent, forma.generate_project, or simulated generation.
• If the connector or OpenCode instance is unavailable, return a typed unavailable state.

Acceptance criteria

• The allowed test user can use browser chat to create and modify a project through OpenCode.
• Existing project ownership and revision controls remain enforced.
• Context questions and server-side build handoffs are removed from OpenCode mode.
• Legacy endpoints are either removed from hosted chat or explicitly disabled behind a documented migration flag.
• Browser messages contain only sanitized OpenCode events.

Related

• Parent: [P0][OpenCode] Hosted project agent bridge between Vercel and mini-PC #454
• [P1][OpenCode] Show OpenCode-generated projects in the Forma OSS GUI #346 OpenCode-generated projects in the GUI
• [P1][Integration] Show Codex, Claude Code, and OpenCode projects in the Forma OSS GUI #344 external-agent projects in the GUI
• [P1][Worker] Provision local Forma execution worker and cloud delivery #443 local worker/cloud delivery