feat(notifications): operational webhook alerts for daemon events

[Aaronontheweb]

Summary

Closes #291

• Adds a webhook notification channel that POSTs structured JSON to configured HTTP endpoints when operational events require human attention
• Supports multiple webhook targets, deduplication within a configurable time window, and retry with exponential backoff
• Emits alerts from McpClientManager (MCP auth expired, server disconnected), FailoverChatClient (provider failover, both providers unreachable), AlertingChatClientDecorator (single-provider unreachable), and SlackChannel (connection failure at startup)
• Adds optional webhook URL input to the init wizard's Exposure step
• Updates netclaw-config.v1.schema.json with the Notifications section
• Fixes pre-existing flaky Reconcile_disables_zombie_oneshot_reminders test (blocking .Result → async await, increased polling timeout)

Alert types

Type	Severity	Trigger
mcp.auth.expired	warning	MCP server needs OAuth re-auth
mcp.server.disconnected	warning	MCP server connection failed
channel.disconnected	warning	Slack connection failed at startup
provider.failover	warning	Primary LLM failed, using fallback
provider.unreachable	critical	All LLM providers failed
provider.auth.expired	—	Reserved for #288

Example config

{
  "Notifications": {
    "Webhooks": [
      { "Url": "https://hooks.slack.com/services/T.../B.../xxx" },
      { "Url": "https://ntfy.sh/netclaw-alerts", "Name": "ntfy" }
    ],
    "DeduplicationWindowSeconds": 300,
    "MaxRetries": 2,
    "TimeoutSeconds": 10
  }
}

Example webhook payload

{
  "alertId": "a1b2c3d4e5f6",
  "type": "mcp.server.disconnected",
  "severity": "warning",
  "summary": "MCP server 'memorizer' connection failed: Connection refused",
  "timestamp": "2026-03-19T14:30:00.000Z",
  "source": "netclaw",
  "hostname": "pi1",
  "context": { "serverName": "memorizer" }
}

Test plan

• dotnet build passes
• All 1,115 tests pass (0 failures)
• 8 new tests for WebhookNotificationService covering delivery, dedup, multi-target, headers, retry, graceful degradation
• Existing tests updated for constructor signature changes
• Flaky reminder test stabilized
• Manual: configure webhook URL in netclaw.json, start daemon with misconfigured MCP server, verify POST received
• Manual: verify no webhooks configured → NullNotificationSink used (log-only)