Add Summary.md to github actions

[j23414]

Context

Realized avian-flu had summary.md statements that makes it much easier to check phylogenetic results. I'm currently inferring the staged phylogenetic paths.

• https://github.com/nextstrain/avian-flu/actions/runs/13641046080

Seems to be enabled with these changes:

• nextstrain/avian-flu@ab3ef2e

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