Fix FreshOpenCode materialization persistence

[danshapiro]

Summary

• emit FreshOpenCode materialization as soon as OpenCode returns the durable ses_* id
• route materialization through the global websocket/client state path so persisted panes and tab snapshots store the durable session ref
• detach stale placeholder subscriptions after materialization
• document the gh account preference in AGENTS.md

Verification

• npm run test:vitest -- test/unit/client/lib/fresh-agent-ws.test.ts test/unit/client/store/tabRegistrySync.test.ts --run
• npm run test:vitest -- --config vitest.server.config.ts test/unit/server/fresh-agent/opencode-serve-adapter.test.ts test/unit/server/ws-handler-fresh-agent.test.ts --run
• npm run typecheck
• git diff --check
• FRESHELL_TEST_SUMMARY='freshopencode materialization second-pass validation' npm run check