Raise spec body character limit from 20k to 100k

Authors can now write spec bodies up to 100,000 characters, up from the previous 20,000-character limit. The higher cap makes room for detailed proposals with substantial Markdown content, and the longer limit is enforced consistently for dashboard and MCP writes, with the form textarea hint updated to match. Comment caps (for spec comments and forage comments) are unchanged at 20,000 characters.