Adopt MCP elicitation JSON Schema format for session input options

- Change schema format from custom types to JSON Schema 2020-12 subset
- Rename configOptions (schema) to configSchema in InitializeResponse
- Add alternatives considered section documenting post-session elicitation
- Reference MCP elicitation spec instead of duplicating schema details

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: chazcb