feat: Add Nagios monitoring provider

[kai-agent-free]

Summary

Implements a Nagios monitoring provider for Keep, closing #3960.

What is Nagios?

Nagios is one of the most widely-used open source monitoring tools for systems, networks, and infrastructure. It monitors hosts and services, alerting when things go wrong and when they recover.

Implementation

Provider (nagios_provider.py)

• Webhook-based integration via Nagios notification commands (curl)
• Maps Nagios service states (OK, WARNING, CRITICAL, UNKNOWN) to Keep severity levels
• Maps Nagios host states (UP, DOWN, UNREACHABLE) to Keep severity levels
• Handles all notification types: PROBLEM, RECOVERY, ACKNOWLEDGEMENT, FLAPPINGSTART/STOP, DOWNTIMESTART/END
• Fingerprints by host_name + service_description for proper deduplication
• Parses Unix timestamps from Nagios into UTC ISO format
• Handles both host and service notifications with appropriate descriptions

Webhook Setup

Detailed documentation for configuring Nagios notification commands to send alerts to Keep, including:

• Service notification command definition
• Host notification command definition
• Contact configuration
• All standard Nagios macros mapped to webhook payload fields

Mock Alerts (alerts_mock.py)

Sample Nagios alert payload for testing.

Pattern

Follows the same pattern as existing webhook-based providers (Checkmk, Zabbix, etc.).

Closes #3960

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[kai-agent-free]

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[kai-agent-free]

I have read the CLA Document and I hereby sign the CLA