fix: fail fast when graphify.serve is launched interactively

[lttlin]

Problem

Running python -m graphify.serve graphify-out/graph.json directly from an interactive terminal feeds the MCP stdio server an empty TTY stream. That bubbles up as a low-level JSON-RPC parse failure instead of explaining that the command must be launched by an MCP client.

Root Cause

graphify.serve immediately enters the stdio transport loop without checking whether stdin/stdout are attached to an interactive terminal. In that mode the MCP runtime tries to parse non-protocol input and reports an EOF/invalid JSON error.

Fix

Add an interactive-stdio guard before starting the MCP server. When stdin or stdout is a TTY, graphify.serve now prints a clear error and exits early instead of surfacing an internal server exception.

Validation

• /tmp/graphify_issue_201/.venv/bin/python -m pytest tests/test_serve.py
• Added coverage for pipe-like stdio and interactive TTY rejection

Closes #201