feat: add OpenCode preset support

[conscientiousness]

Summary

• add a dedicated Dockerfile.opencode image that bundles openab with the OpenCode CLI
• add an opencode Helm preset and wire it into the image/command/args helpers and Helm NOTES
• align the OpenCode runtime with the node:22-bookworm-slim base image by using /home/node and the existing node user
• update CI, release docs, and config examples to cover OpenCode install and auth flows

Validation

• cargo +1.88.0 test
• helm template openab charts/openab --set discord.botToken=dummy --set-string 'discord.allowedChannels[0]=1234567890' --set agent.preset=opencode
• helm lint charts/openab --set discord.botToken=dummy --set-string 'discord.allowedChannels[0]=1234567890' --set agent.preset=opencode
• docker build -f Dockerfile.opencode -t ghcr.io/openabdev/openab-opencode:test .
• docker run --rm --entrypoint sh openab-opencode:test -lc 'whoami && echo $HOME && command -v openab && command -v opencode'
• docker run --rm --entrypoint opencode openab-opencode:test acp --help
• Docker Desktop kubeadm deployment with image.pullPolicy=Never
• kubectl exec -it deployment/openab -- opencode auth login
• Discord end-to-end flow: new mention creates a thread, OpenCode handles file creation, and /home/node/smoke.txt persists after kubectl rollout restart deployment/openab

[conscientiousness]

I did a full local smoke test and this is working end-to-end for me. I verified the image build, Docker Desktop k8s deploy, OpenCode login, Discord replies in new threads, agent file operations.

[thepagent]

Impressive!

[thepagent]

depend on #54

[neilkuan]

thanks @conscientiousness can not wait see this feature support,
#54 was merged.
please rebase main branch, thanks~

[conscientiousness]

Rebased onto the latest \main and aligned this PR with the post-#54 chart structure. I also re-ran the local end-to end validation for the OpenCode preset. Please take another look when you have a chance.

[thepagent]

close in favor of #258