Add existing MCP client bridge

[imran-siddique]

Closes #491. Closes #510.

Adds a stdio bridge that existing MCP clients can launch from their standard mcpServers configuration. Tokens are read from an environment variable, stdout remains strict JSON-RPC, failures are fail closed, and the tutorial states plainly that the local bridge is outside the attested boundary and is removable by the user.

Evidence:

• design issue design: stdio client bridge for existing MCP clients #510 existed before implementation
• official MCP Python client launched the bridge as a subprocess
• client initialized, listed tools, and called a tool through a real cMCP HTTP server
• the real cMCP audit chain contained the tool call
• focused bridge tests: 5 passed
• full suite: 1161 passed, 24 skipped
• Ruff, mypy, Bandit, and git diff check passed