v0.4.4 — redact secrets on every MCP tool result

Security fix: the MCP surface now redacts secrets the same way export does.

• rungraph export already blocked on high-confidence secrets; rungraph mcp did not. A tool result travels in the calling agent's API request and is written into that session's own transcript, so a key read once in one run came to rest in a second one.
• Redaction runs at the single callTool choke point, which covers node labels as well as payloads — a prompt label at the 80-char snippet cap could carry a complete AWS key, so find_nodes leaked without anyone fetching a payload. Every redacted result reports how many values it replaced.
• walkStrings / redactTree live in src/secrets.js, shared by both boundaries. The dashboard stays verbatim by design: it renders over loopback to your own browser, where seeing a key is how you rotate it.
• The secrets fixture grew a fifth location (a node label) to pin the behaviour; the site FAQ names both boundaries.