Fixes #6942

Summary

• Enable click-to-edit for queued user messages using the standard dialog flow
• Add queued edit/delete controls wired to part update/delete

Note: The modal is centered, in the video it's slightly off because I cropped the video to remove the sidebar from showing.

Notes on conventions

• Uses the existing dialog pattern (dialog.replace + DialogPrompt) in packages/opencode/src/cli/cmd/tui/routes/session/index.tsx
• Reuses existing SDK part update/delete methods so sync events flow through the standard bus

Verification

• Manual: queued messages can be edited via modal and removed via delete

UI