[feature] Add optional bearer auth to the MCP HTTP gateway #13

Problem this solves
The gateway (localhost:3100) and Ghidra headless (localhost:8089) currently have no authentication. The README already warns that exposing 0.0.0.0:3100 grants arbitrary command execution via shell__exec. Loopback-only binding is safe by default, but there's no safety net if someone deploys this on a shared/cloud VM or changes the port mapping.

Proposed solution

• Add a GATEWAY_AUTH_TOKEN env var (mirroring GHIDRA_MCP_AUTH_TOKEN), empty by default for local dev.
• When set, require Authorization: Bearer <token> on all /mcp requests.
• Document in README: "required if binding to a non-loopback interface."

Acceptance criteria

• Gateway rejects requests without valid token when GATEWAY_AUTH_TOKEN is set
• No behavior change when the var is unset (backward compatible)
• README updated with a warning + example

Affected component(s)

• Container / Dockerfile
• MCP gateway
• Ghidra bridge (bridge_mcp_ghidra.py)
• Shell server (shell-mcp.py)
• r2mcp / radare2 tools
• angr MCP
• Agents (.claude/agents/)
• CI / build
• Docs

Additional context
Relevant links, upstream issues, or prior discussion.