Document that vault_patch does not synthesize blank-line separators

The patch engine only preserves a blank line at a target boundary if one
already existed there before the patch; it never inserts one that wasn't
already present. This surprised an LLM caller, which produced several
instances of heading/content corruption where its patched text got glued
onto adjacent content with no separating newline. The underlying behavior
is intentional and covered by existing markdown-patch tests, so the fix
is to document the contract (across the MCP tool description, OpenAPI
docs, and README) rather than change behavior.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>