sessions - adopt modal editor support from core (#296585)

* sessions - adopt modal editor support from core

* Update src/vs/workbench/services/editor/test/browser/modalEditorGroup.test.ts

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update .github/skills/agent-sessions-layout/SKILL.md

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update src/vs/workbench/services/editor/common/editorGroupFinder.ts

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>