[MCP] Extract config resolution into a tested config module

[manoahLinks]

Summary

Network, URL, and contract-id resolution runs as top-level side effects in mcp/src/index.ts, including two process.exit(1) calls, which makes the config path effectively untestable.

Scope

• Keep the implementation aligned with the existing MindVault architecture.
• Add or update automated tests.
• Update docs when the change affects contributors, deployers, or agent users.

Acceptance criteria

• Config resolution moves to a dedicated module that returns a typed config object or a validation error instead of exiting.
• The entrypoint keeps the current fail-fast behaviour and messages, with tests covering valid and invalid environments.
• Failure cases are deterministic and safe to show in issue-driven contributor work.
• Relevant local tests pass.

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