feat: moves prompt history to be project scoped instead of global

[spoons-and-mirrors]

Summary

Currently, prompt history is globally scoped and shared amongts all user projects. This PR changes this and makes prompt history project scoped.

Before:

• ~/.local/state/opencode/tui (global)

After:

• ~/.local/share/opencode/project/{repo-name}/prompt-history (project scoped)

If not in a git repo, falls back to:

• ~/.local/share/opencode/project/global/prompt-history

Resolves https://github.com/sst/opencode/issues/2067