fix: restore OpenCode text responses

[kapelame]

What changed

OpenCode 1.17.18 emits assistant output as a top-level text event with the content under part.text. The daemon previously ignored that event and persisted a completed run with no output.

This PR:

• translates the current text event into Parsar delta/final output;
• captures usage from the current step_finish payload, including nested cache counters;
• keeps the existing message.part.delta format supported.

Scope

Only the OpenCode event parser and its regression tests are changed. No Claude Code, model, MCP, API, database, or storage changes are included.

Validation

• go test ./apps/parsar-daemon/internal/agent/opencode ./server/internal/connector/agentdaemon
• go build ./apps/parsar-daemon/cmd/parsar-daemon
• Local end-to-end SSE verification returned delta: OK and done: OK.