Add COPY method for vault files (#303)

Mirrors the existing MOVE implementation: a REST COPY method on
/vault/{path} and a matching vault_copy MCP tool. Unlike MOVE, the
source file is left in place; Allow-Overwrite and trailing-slash
destination semantics match MOVE exactly. Uses Obsidian's public
Vault.copy() API, which bumps minAppVersion to 1.8.7.

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>