feat(mcp): userPrompt optional on propose_file_access

userPrompt was required, which added friction agents routinely skipped or
filled with placeholder text. The audit value isn't worth blocking the
lock on a missing field. Made optional; description clarifies the trade.

Also tightened the description: cut redundant prose, kept the imperatives
and the unique constraint (file-only, not directory locks), which agents
miss most often.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>