Drop the stale ScottPlot.Avalonia ignore and Avalonia 11 references

[erikdarlingdata]

Summary

Avalonia 12 merged in #549, but three places still describe the old state.

• The Dependabot ignore for ScottPlot.Avalonia >= 5.1.59 blocks every future ScottPlot.Avalonia update. It held ScottPlot at 5.1.58, the last build for Avalonia 11, until the migration. The pin is 5.1.59 now, so the entry only stops updates. This PR removes it. 5.1.59 is the newest version on nuget.org today, so no Dependabot PR follows right away.
• The comment on the Avalonia* major ignore says the migration is parked on upgrade/avalonia-12. That branch is gone from the remote. The ignore itself stays: it still keeps Avalonia majors out of Dependabot. The new comment says why a major is done by hand. It also records the NU1605 trap for the next major, which the removed entry used to document.
• CONTRIBUTING.md called the app "an Avalonia 11 desktop app". The maintenance skill named upgrade/avalonia-12 as an example of a branch to keep.

Dependabot reads its config from the default branch, main. So this change takes effect with the next release merge, not when it lands on dev.

Changes

• .github/dependabot.yml: remove the ScottPlot.Avalonia ignore, and rewrite the Avalonia* comment. The other ignore rules do not change.
• CONTRIBUTING.md: says Avalonia 12, not Avalonia 11.
• .claude/skills/maintenance/SKILL.md: the parked-branch example no longer names a deleted branch.

Test Plan

• The edited dependabot.yml parses. 9 ignore entries remain: all of the old ones except ScottPlot.Avalonia.
• Directory.Packages.props pins Avalonia 12.1.3 and ScottPlot.Avalonia 5.1.59.
• Once this reaches main, the next ScottPlot.Avalonia release shows up in the grouped Dependabot PR.

Generated with Claude Code

https://claude.ai/code/session_019tS6P95Dtzs4aeMpb1xqzX

[claude]

Reviewed: config/docs only (dependabot.yml, CONTRIBUTING.md, maintenance skill). Claims check out: Directory.Packages.props pins Avalonia 12.1.3 and ScottPlot.Avalonia 5.1.59, and removing the >= 5.1.59 ignore is correct now that the pin is at that version. No code, version-bump, T-SQL, or warning concerns. No findings.