Remove legacy public-ingress alert issue URL model

[shiny-code-bot]

Finish Line

Public ingress alert routing no longer uses lane alert_issue_url fields or standing issue comments.

Current Status

State: Planned after replacement incident and policy paths exist.
Next action: Remove legacy fields and compatibility only after #981 and #982
provide the new source of truth.
Blocked by: #981 and #982.
Waiting for: Replacement incident and notification policy records.
Last verified: 2026-05-29.

Scope

• Remove alert_issue_url from ProductPublicIngressMonitoringPolicy.
• Remove notification_key and notification_sent from new observation writes
  and contracts once migration is complete.
• Remove build_github_issue_notifier, public_ingress_alert_body, and inline
  notifier callbacks from the monitor workflow.
• Update docs/tests/import material to describe the replacement model.
• Preserve historical records safely without treating old fields as authority.

Acceptance Criteria

• Production alert routing cannot be configured through lane text fields.
• Public-ingress monitor code has no standing issue comment path.
• Historical observation payloads remain readable or safely tolerated.
• Docs explicitly state that notification routing is DB/service-backed except
  explicit bootstrap/import material.

Relationships

Parent: #929.
Blocked by: #981, #982.