Raise spec body cap from 20k to 100k characters

The character limit for spec bodies is raised from 20,000 to 100,000 characters, giving you much more room for detailed proposals with substantial Markdown content. The body column already supports the larger size, so this is purely a validation bump: no migration is required, and the dashboard and MCP write paths now enforce the new cap, with the form textarea’s hint updated to match. Comment caps are unchanged.