Add MCP client command validation and security documentation

[bug-ops]

Context

Security audit (2026-02-15) found that zeph-mcp/src/client.rs:38-69 spawns child processes from user config without command validation. Also lacks SSRF protection for URL-based MCP connections.

Severity

Low — config is admin-controlled, not externally modifiable.

Tasks

1. Documentation: Add docs/src/security/mcp.md covering:

   • Safe MCP server configuration
   • SSRF risks with URL-based connections
   • Recommended command allowlists

2. Optional validation: Consider command allowlist for MCP binaries:

const ALLOWED_MCP_BINARIES: &[&str] = &["node", "python", "python3", "npx"];

3. SSRF protection for URL-based connections:
   • Block private IP ranges (10.x, 172.16-31.x, 192.168.x, 127.x)
   • Block link-local and metadata endpoints (169.254.x)

Acceptance criteria

• MCP security documentation page exists in mdBook
• SUMMARY.md updated
• SSRF protection implemented for URL-based MCP connections