[FEATURE]: /simplify — parallel multi-agent code sweep to deduplicate and refactor

Requested feature

Add a /simplify slash command (inspired by Claude Code's /simplify) that runs a parallel sweep with multiple agents to refactor and clean up redundant code.

Why

openCode has subagents (packages/opencode/src/agent) but no built-in orchestrated multi-agent code-quality pass. Large refactors currently require manual review or external tooling.

Proposed behavior

• /simplify dispatches independent subagents (one per module/area) to detect dead code, duplication, and over-engineering.
• Findings are aggregated into a review; changes applied behind a diff that can be reviewed before commit.
• Leverages the existing subagent task mechanism and /diff viewer.

Benefits

• Parallel cleanup that scales with the repo.
• Safe aggregation + diff review of AI-proposed refactors.