[Repo Assist] improve: extract FormatAge into ModelFormatting, removing duplicate logic

[github-actions]

🤖 This PR was created by Repo Assist, an automated AI assistant.

Summary

GatewayNodeInfo and SessionInfo both implemented identical age-formatting logic independently:

if (delta.TotalSeconds < 60) return "just now";
if (delta.TotalMinutes < 60) return $"{(int)Math.Round(delta.TotalMinutes)}m ago";
if (delta.TotalHours < 48) return $"{(int)Math.Round(delta.TotalHours)}h ago";
return $"{(int)Math.Round(delta.TotalDays)}d ago";

This PR extracts that logic into a single ModelFormatting.FormatAge(DateTime) helper, consistent with the existing ModelFormatting.FormatLargeNumber pattern.

Changes

• ModelFormatting — new FormatAge(DateTime timestampUtc) method
• SessionInfo.AgeText — delegates to ModelFormatting.FormatAge(UpdatedAt ?? LastSeen)
• GatewayNodeInfo.FormatAge — delegates to ModelFormatting.FormatAge(timestampUtc)

No behaviour change. Future age-formatting tweaks (e.g. adding weeks, locale) now only need one edit.

Test Status

• ✅ Build succeeded (0 errors, 0 warnings)
• ✅ Shared tests: 525 passed, 20 skipped, 0 failed
• ✅ Tray tests: 99 passed, 0 failed
• ℹ️ All existing SessionInfoAgeTextTests and GatewayNodeInfoTests.DetailText_LastSeen_* tests pass unchanged, confirming no behaviour regression.

Generated by Repo Assist · ◷

To install this agentic workflow, run

gh aw add githubnext/agentics/workflows/repo-assist.md@cbb46ab386962aa371045839fc9998ee4e97ca64