Add slash command and agent mention autocomplete system

Implement intelligent autocomplete for slash commands and agent mentions:
- SuggestionMenu component displays up to 5 filtered suggestions
- Slash commands (/) trigger command palette with fuzzy matching
- Agent mentions (@) scan local .agents directory and suggest by display name
- Arrow keys, Tab/Shift+Tab navigate; Enter inserts selection
- MultilineInput accepts onKeyIntercept callback for menu key handling
- Parsing functions detect trigger contexts and track cursor position
- Filtering prioritizes prefix matches, then includes substring matches

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Author: brandonkachen