feat: Implement MachineReconciler with SSH connectivity checks

[kitsunoff]

⚠️ TDD MANDATORY

Tests MUST be written BEFORE implementation code. No exceptions.

All SSH scenarios, failure modes, and state transitions must have tests written FIRST.

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Description

Implement MachineReconciler that handles SSH connectivity checks and hardware scanning.

Tasks

Test First: SSH Scenarios

• Test: SSH connection refused → Discoverable=False, reason=SSHFailed
• Test: SSH connection timeout → Discoverable=False, reason=SSHFailed
• Test: SSH authentication failed → Discoverable=False, reason=SSHFailed
• Test: DNS resolution failed → Discoverable=False, reason=SSHFailed
• Test: SSH key secret not found → Discoverable=False, reason=CredentialsMissing
• Test: SSH connection success → Discoverable=True, Ready=True
• Test: Machine deletion blocked by config → DeletionBlocked=True

Core Reconciler

• Implement MachineReconciler struct with dependencies
• Implement Reconcile() with observedGeneration updates
• Implement SetupWithManager() with Secret watches
• Add finalizer handling for deletion

SSH Interface

• Define SSH Client interface for testability
• Implement real SSH client using golang.org/x/crypto/ssh
• Implement mock SSH client for testing
• Handle key-based and password-based authentication
• Implement connection timeout handling

Discovery Logic

• Implement checkDiscoverable() with SSH connection test
• Update Discoverable condition based on result
• Handle various failure modes (connection refused, timeout, auth failed, DNS)
• Emit events for state changes

Hardware Scanning

• Write tests for hardware scan parsing
• Implement hardware scan execution via SSH
• Parse scan results into hardwareFacts
• Store nixFacterResult as raw JSON
• Schedule periodic rescans

Secret Watches

• Write tests for Secret watch triggers
• Register field indexes for SSH key/password secrets
• Implement findMachinesForSecret() mapper
• Add predicate to filter only data changes

State Machine

Undiscoverable <---> Discoverable
      |                    |
      v                    v
   Deleting            Deleting
      |                    |
      v                    v
   Deleted              Deleted

Reference

See docs/kubebuilder-migration-analysis.md sections 9, 14, 23, 24, 26.