ci: deduplicate e2e workflows

[TaylorMutch]

Summary

Deduplicates PR E2E image builds by routing standard, GPU, and Kubernetes E2E through Branch E2E Checks. The standard E2E suite runs on test:e2e; GPU remains separately gated by test:e2e-gpu while sharing the supervisor build when both labels are present.

Related Issue

N/A

Changes

• Consolidated PR E2E image builds into Branch E2E Checks with multi-arch SHA-tagged images.
• Kept GPU E2E separately gated by test:e2e-gpu and the standard E2E suite gated by test:e2e.
• Moved Kubernetes E2E into a reusable workflow_call and called it from Branch E2E Checks.
• Added independent suite result jobs so required statuses stay separate inside the shared workflow.
• Removed standalone PR GPU/Kubernetes workflow entrypoints and updated gate/status docs.
• Preserved the Branch Checks Rust cache-on-failure fix.

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (workflow-only change; not run locally)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)

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