feat(tui): add auto_scroll configuration setting

[alexx-ftw]

Summary

Adds a new tui.auto_scroll configuration option to control the sticky scrolling behavior in the TUI session view. This addresses the request to prevent auto-scrolling when new messages arrive.

Changes

• Added auto_scroll boolean to TUI config schema in packages/opencode/src/config/config.ts
• Updated packages/opencode/src/cli/cmd/tui/routes/session/index.tsx to respect the auto_scroll setting (defaults to true).

Fixes #7648