fix(web): hide opencode's plan agent when legacy plan mode is off - #6420

💭 This diff looks like React/TypeScript frontend code without any Effect services involved, so I should check whether any touched files actually use Effect conventions. Since this appears to be UI/settings code, I suspect there won't be relevant findings, but I'll grep the changed files for "effect" to confirm before ruling it out. No Effect service definitions were touched here—this PR is pure React/UI, so there are no service convention violations to flag. I'm reconciling the instruction to output exactly "All clear" with the requirement to still produce valid JSON per the schema, so I'll fit "All clear" into the title/summary field while treating this as a success case since there are no findings.