feat(tui): add rules mode to save instructions

[sachnun]

Summary

• Add rules mode (triggered by # prefix) to save instructions to project AGENTS.md
• Similar to Claude Code's memory feature where # prefix saves to instruction file

Why

• Convenient way to add persistent rules/instructions without manually editing AGENTS.md

Limitations

• Only supports project-level AGENTS.md (not global)
• No edit/delete functionality for saved rules

Relates to #1217