fix: restore OpenCode text responses - #239

What changed

OpenCode emits assistant output from opencode run --format json as top-level text events with content under part.text. The daemon previously ignored those events and could persist a completed run with no output.

This PR:

• translates current text events into Parsar delta and final output;
• accumulates usage from each step_finish payload, including nested cache counters;
• keeps the existing message.part.delta text format supported.

Scope

Included:

• the OpenCode event parser;
• focused parser regression tests.

Excluded:

• other OpenCode event types and pre-existing error presentation;
• Claude Code, Codex, Pi, model, MCP, API, database, storage, and frontend behavior.

Validation

• go test ./apps/parsar-daemon/internal/agent/opencode ./server/internal/connector/agentdaemon
• go build -o /tmp/parsar-daemon-pr239 ./apps/parsar-daemon/cmd/parsar-daemon
• make check (passed; the repository check skipped its Postgres migration smoke test because Docker is unavailable)
• two independent blind-review rounds plus one final convergence review; final review: no findings