fix(opencode): add missing NOTES.txt and RELEASING.md updates

[thepagent]

Summary

PR #258 (feat: add OpenCode as ACP agent backend) was merged without updating two files that Cursor PR #301 correctly includes:

• charts/openab/templates/NOTES.txt — missing cursor-agent login-style auth instructions for opencode auth login
• RELEASING.md — image variant list and count not updated to include openab-opencode

Expected

NOTES.txt should include an opencode branch in the agent auth instructions (matching the pattern for kiro, claude, gemini, cursor).

RELEASING.md should list ghcr.io/openabdev/openab-opencode in the image variants section.

Reference

• PR feat: add OpenCode as ACP agent backend #258 (merged, missing these two files)
• PR feat: add Cursor Agent CLI support #301 (Cursor — correctly includes both files as reference)

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