Part of #1159

Add per-server tool policy enforcement to MCP multi-server client.

Crates: zeph-mcp
Priority: P1 (pre-1.0)

Tasks:

• McpPolicy struct: server_name → allowed tools, denied tools, rate limits
• PolicyEnforcer middleware: intercept tool calls, check policy, reject violations
• Config: [[mcp.servers.policies]] TOML sections per server
• Audit log: violations via tracing::warn! with structured fields (server, tool, action, reason)

Files: crates/zeph-mcp/src/policy.rs (new), crates/zeph-mcp/src/client.rs, crates/zeph-core/src/config/types.rs