fix(trace-opencode): lazy-init SessionState in chat.message for API-created sessions

[ekeith (evanmkeith)]

Summary

Sessions created via the OpenCode HTTP API (POST /sessions) were silently dropping all trace data. OpenCode publishes session.created internally but does not deliver it to
plugins for API-created sessions. Since the plugin initializes SessionState exclusively in the session.created handler, every subsequent hook (chat.message,
message.part.updated, message.updated, tool.execute.*) found no state and returned early — resulting in complete, silent data loss.

Changes

• src/tracing.ts — chat.message hook: change const state → let state; replace the if (!state) { return } early-exit with lazy initialization that creates a root span and SessionState on demand, identical to the session.created root-session path
• src/tracing.ts — session.created handler: add a guard (if (sessionStates.has(sessionKey)) return) so a late-arriving session.created event doesn't overwrite an already lazy-initialized state and orphan the root span

Testing

• API-created session: POST /sessions → send message via API → spans appear in Braintrust
• Interactive session (regression): open OpenCode normally → send message → traces unchanged
• Bump version to 0.0.6 in package.json

Notes

startTime for lazily-initialized sessions is set at the first chat.message rather than true session creation. This is acceptable — API-created sessions have no meaningful
"open" period before the first message.

Fixes BT-4649.