[CLI] Gate mcp-dev-wrapper DEBUG / noisy stderr behind env flag

[cloud-nitrostack]

Reduce noisy stderr from mcp-dev-wrapper during stdio MCP dev

Summary: Dev wrapper logs hot-reload and debug info to stderr, which is painful when debugging stdio MCP.

Where to change: typescript/packages/cli/src/mcp-dev-wrapper.ts.

Acceptance criteria:

• Debug lines (e.g. MCP_SERVER_PORT) behind NITROSTACK_DEBUG or DEBUG namespace.
• Real failures remain obvious on stderr.

Suggested labels: dx, cli, good first issue

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Source: docs/STARTER_ISSUES_REVIEW.md — edit in-repo first, then sync GitHub issues if needed.