fix(trace-opencode): support opencode 1.14.x delta event stream

[Evan Burrell (evan-burrell)]

Closes #9. (Re-opened from #10 on the renamed branch feature/opencode-1-14-event-stream.)

Summary

• opencode 1.14.x dropped message.updated / message.part.updated from the plugin event bus and now streams via message.part.delta + session.idle. The existing LLM-span path never fired against 1.14.x → Braintrust showed LLM calls: 0 for every turn.
• Add a message.part.delta handler that accumulates text-field deltas per messageID, then synthesize an LLM span on session.idle when the old message.updated path didn't already emit one. Old path stays intact for opencode <1.14; a per-turn guard prevents double-emit when both fire.
• Provider/model captured from chat.message is reset unconditionally each turn so Turn N+1 doesn't inherit Turn N's model identity. Empty-response turns (no deltas at all) skip the LLM span entirely — absence is the signal. Non-text delta fields (future tool_input streaming) are filtered so they can't corrupt assistant text.
• Shared synthesis lives in src/delta-synthesis.ts so the test-driving EventProcessor and the production hook stay in sync.

Test plan

• bun test — 92 pass / 0 fail (8 new tests added for the 1.14.x flow: delta-only synthesis, transition case, system-prompt prepend, no-model leak across turns, empty-response skip, non-text field filter, epoch-0 start preservation, turn-span output fallback)
• npx tsc --noEmit — clean
• npm run check — clean (biome lint + format)
• bun build src/index.ts --outdir dist --target bun --format esm — clean
• Manual smoke against opencode 1.14.48 in cloudflare/sandbox: confirm LLM calls: >= 1 per turn in the Braintrust dashboard
• Smoke test against opencode <1.14 (if available): confirm message.updated-driven path still emits with token counts and the synthesis path stays dormant