fix(opencode): stream persistent HTTP sessions

[zmlgit]

Summary

• Add an optional OpenCode connection_url mode that talks to opencode serve over HTTP and keeps /event SSE connected before prompts are sent.
• Stream assistant text/reasoning/tool/status events from SSE while the synchronous /session/{id}/message request is still running, preserving progress updates and background callbacks.
• Forward OpenCode permission/question requests back through cc-connect, and ignore user prompt text parts so replies do not echo the user's own message.

Why

opencode run --attach is short-lived in cc-connect. When the client process exits at the end of a turn, OpenCode server-side background callbacks can be lost. The OpenCode CLI already subscribes to /event SSE internally; this patch mirrors that behavior in cc-connect when connection_url is configured.

Related: #1534, especially #1534 (comment)

Test Plan

• GOCACHE=/tmp/cc-connect-go-cache go test ./agent/opencode -count=1
• GOPATH=/tmp/cc-connect-go GOMODCACHE=/tmp/cc-connect-go/pkg/mod GOCACHE=/tmp/cc-connect-go-cache go build ./...
• Live local validation against opencode serve: OPENCODE_LIVE_URL=... OPENCODE_LIVE_USERNAME=... OPENCODE_LIVE_PASSWORD=... OPENCODE_LIVE_SEND=1 go test ./agent/opencode -run TestOpencodeHTTPModeLiveServer -count=1 -v
• Manual Feishu validation with the rebuilt local service: streaming works, background callbacks survive, and user prompts are no longer echoed in bot replies.