refactor(opencode): move tui state io to AppFileSystem

[kitlangton]

Summary

• move TUI prompt history, frecency, stash, KV, local model state, editor, clipboard, and export writes off Filesystem
• use direct Bun and existing Node file APIs in these UI-facing modules instead of routing them through AppRuntime
• keep append-only paths and existing TUI behavior intact while removing the direct util dependency from this slice

Verification

• bun typecheck from packages/opencode currently fails in this environment because tsgo is not installed (tsgo: command not found)