feat: add configurable prompt history max size (experimental)

[DanielFGray]

• Add prompt_history_max_size to TUI config with default value of 50
• Update history provider to use config value instead of hardcoded constant
• Allow users to customize how many prompts are kept in memory
• Config can be set in opencode.json: { "tui": { "prompt_history_max_size": 200 } }