feat(mcp): implement MCP_LOG_LEVEL stderr logging (#558)

[developmentwithparth1311]

Resolves #558 by introducing an MCP_LOG_LEVEL controlled logger in the @mindvault/mcp package.

Summary of Changes

• Logger Module (logger.ts): Implemented a dedicated logger supporting debug, info, warn, and error priorities with fallback to info.
• Stdio Safety: Guaranteed all log outputs are written exclusively to process.stderr (using util.format) to avoid corrupting stdout MCP transport messages.
• Refactoring (index.ts): Replaced ad-hoc console.error calls with appropriate logger.* levels. Resolved a pre-existing setupGracefulShutdown compilation blocker.
• Unit Testing (logger.test.ts): Added comprehensive test coverage verifying priority filtering, default fallbacks, and stdout isolation.
• Documentation: Added MCP_LOG_LEVEL details to README.md and mcp-client-configs.md.

Verification

• Ran pnpm --filter @mindvault/mcp test — all logger unit tests pass cleanly.
• Verified the package compiles and builds without any TypeScript errors.