feat(mcp): expand MCP server with learn, status, session_list tools

[magicpro97]

User Story

As an AI agent using MCP, I want to write knowledge entries and query session status so that sk becomes a full read/write knowledge tool in agentic loops.

Problem

The MCP server exposes only 3 read tools. Agents cannot write mistakes/patterns or check health.

Acceptance Criteria

• learn MCP tool: category, title, description, tags → persists entry, returns receipt with id
• status MCP tool: returns JSON health snapshot (session count, entry count, watcher status)
• session_list MCP tool: returns recent sessions (id, summary, created_at, label)
• All 3 tools wired in _handle_tools_call() dispatch table
• Tests in test_fixes.py

Priority: HIGH