C1: persistent SSH connection pool, C2: TextFSM structured output

C1 — Persistent connection pool:
- Added CISCO_SWITCH_PERSISTENT_CONNECTION setting (default false)
- Added get_connection() / close_connection() with thread-safe locking
- run_command(), run_config_commands(), and test_connection() all honour persistent mode
- Added cisco_switch_reconnect() tool to force a fresh connection

C2 — TextFSM structured output:
- Added run_command_structured() that tries TextFSM, falls back to raw JSON-wrapped text
- Updated 6 diagnostic tools to return structured JSON:
  show_version, show_interfaces_status, show_mac_address_table,
  show_vlan_brief, show_ip_route, show_cdp_neighbors
- Added cisco_switch_test_connection_structured() tool
- Added cisco_switch_get_mcp_config() alias
- Added .gitignore for __pycache__ and build artifacts

.gitignore

@@ -0,0 +1,7 @@
+__pycache__/
+*.pyc
+*.pyo
+*.egg-info/
+dist/
+build/
+.eggs/