production-image-deploy: refine validation and Slack links

[jwmossmoz]

Summary

Improve production image validation and Slack changelog links.

• Permit a failed validation check only when it matches a confirmed, pre-existing Gecko intermittent on the current production image.
• Keep candidate-only failures and worker or image failures as rollout blockers.
• Require evidence for each accepted failure in the PR description or a linked report.
• Use named Markdown links when sending a changelog through the Slack connector.
• Read the sent message back and verify each link label and target.
• Add a connector-safe Ubuntu template with the deployment and SBOM links.

Slack link root cause

The Slack connector parsed each bare URL together with text from the next line. This produced invalid links and 404 responses.

Validation

• Sent the Ubuntu changelog with the new format: verified Slack DM
• Read-back verification passed for the PR, build, and all five SBOM links.
• skills-ref: passed
• skill-validator: passed with no warnings
• skill-check: 85/100, no errors, one advisory warning
• git diff --check: passed

The existing skill-size and missing-evaluation advisories remain.