fix(ci): recognize supported runner labels during migration

[flcrom]

The migration analyzer was flagging native Depot runners and mapped Ubuntu runners as custom labels. Reuse the mapper's ClassifyLabel so analysis agrees with the generated workflow, and fix the runner spelling in the warning.

Adds regression coverage for native and mapped labels, expressions, case/whitespace, and unknown or mixed labels. Unknown and self-hosted labels still warn.

Tested: full Go suite and build with Go 1.25.14, golangci-lint v2.4.0, focused race tests, and npm formatting/type checks.

Fixes #578

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Note

Low Risk
Narrows false-positive warnings in CI migration analysis only; behavior is covered by new tests and does not change runtime workflow execution.

Overview
The migration compatibility analyzer no longer flags supported runs-on values as custom labels. hasCustomRunsOn now delegates to migrate.ClassifyLabel so Depot-native (depot-*), standard GitHub Ubuntu labels, and ${{ … }} expressions match the same rules as workflow migration; unknown, Windows/macOS, self-hosted, mixed, and legacy underscore Depot names still raise a partial runs-on (custom labels) issue.

User-facing copy in JobFeatureRules now recommends depot-ubuntu-latest (hyphenated) instead of depot_ubuntu_latest.

TestAnalyzeJobsRunnerLabels adds table-driven regression coverage for the above label cases.

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