tweak: improve motel observability for session flows

[kitlangton]

Summary

• default local OTLP export to the motel dev server through the existing Effect observability layer and restore log-to-span-event correlation
• add structured span annotations and logs in the main session, processor, and tool execution flows so motel queries surface session IDs, tool calls, and model context
• keep WriteTool formatting inside the shared Effect runtime so child spans like Format.file can stay nested under tool execution

Verification

• bun run typecheck
• bun test test/tool/write.test.ts
• curl http://127.0.0.1:27686/api/services
• curl "http://127.0.0.1:27686/api/traces?service=opencode&limit=5&lookback=1h\"
• curl "http://127.0.0.1:27686/api/logs?service=opencode&limit=5&lookback=1h\"
• curl "http://127.0.0.1:27686/api/logs/stats?groupBy=severity&agg=count&service=opencode\"