Remove OMO OpenCode settings

[CollieIsCute]

Summary

• remove the OhMyOpenAgent/OpenCode plugin from opencode.json
• delete the OMO agent/category override config
• add a chezmoi remove marker so apply removes the old OMO config file
• remove OMO/Sisyphus cache ignore entries from .gitignore

Verification

• git grep -n -i -E 'oh[-_ ]?my[-_ ]?open|openagent|(^|[^[:alpha:]])omo([^[:alpha:]]|$)|sisyphus|prometheus|hephaestus|ultrawork|ralph|boulder|momus|metis' -- .
• rg -n -i "OhMyOpenCode|OhMyOpenAgent|oh-my-opencode|oh-my-openagent|openagent|.omo|.sisyphus|sisyphus|prometheus|hephaestus|ultrawork|ralph|boulder|momus|metis" .
• chezmoi execute-template < home/dot_config/opencode/opencode.json.tmpl | jq empty
• chezmoi --source /tmp/chezmoi-remove-omo --destination /tmp/chezmoi-omo-qa --refresh-externals=never apply --force /tmp/chezmoi-omo-qa/.config/opencode
• test ! -e /tmp/chezmoi-omo-qa/.config/opencode/oh-my-openagent.json
• chezmoi --source /tmp/chezmoi-remove-omo --destination /tmp/chezmoi-omo-qa --refresh-externals=never verify /tmp/chezmoi-omo-qa/.config/opencode
• opencode --help