feat(providers): add mcp category for provider profiles

Problem Statement

Provider profiles use a category field to classify services. Current valid values: inference, source_control, agent, other, data, messaging, knowledge. There is no mcp category for MCP tool server profiles.

As MCP tool servers become a common integration pattern for agents, MCP-specific profiles need a dedicated category so developers can filter and discover them via openshell provider list-profiles.

Proposed Design

Add mcp to the valid category values in crates/openshell-providers/src/profiles.rs.

This is a one-line addition to the category enum. No schema changes, no behavioral changes.

Agent Investigation

• Category enum parsed in crates/openshell-providers/src/profiles.rs:825 — unknown categories produce "unsupported provider profile category" error
• McpOptionsProfile struct exists but isn't used in any built-in profile YAML
• MCP L7 protocol (L7Protocol::Mcp) already recognized by the supervisor relay

Checklist

• Searched for existing issues — no issue tracks adding an mcp category
• Minimal scope — one enum value addition

Start in crates/openshell-providers/src/profiles.rs around the category enum at line 825. Review the existing valid categories and the unsupported-category error, then add the requested MCP category. Done means MCP provider profile categories are accepted without schema or behavioral changes.