fix: preserve optional supersede confidence when omitted

[timeleft--]

Problem

The MCP supersede input schema marks confidence optional, but the handler forwards arguments.get("confidence") as an explicit None. TrailManager.supersede then uses that None instead of the original thought confidence, and Pydantic rejects the successor before any mutation.

Expected

An MCP supersede call without confidence should retain the original thought confidence, matching the direct TrailManager.supersede behavior.

Acceptance

• Add an MCP-level regression test calling supersede without confidence.
• Assert an atomic successor/backlink is created and the successor retains the original confidence.
• Preserve explicit numeric confidence overrides.

Context

Observed during WisdomHelm #9 dedicated gateway acceptance. The deployment passed when the client sent an explicit numeric confidence; no secret, credential, profile, or tunnel identifier is involved.