[Repo Assist] [chore]: verify KSail compatibility with ArgoCD v3.4 stable and update embedded client

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🤖 This issue was created by Repo Assist from the weekly roadmap.

User Story

As a KSail user running ArgoCD-based GitOps,
I want KSail to embed the ArgoCD v3.4 stable client,
So that I can benefit from v3.4 performance improvements and new features (ApplicationSet SOPS parity is already documented).

Context

ArgoCD v3.4.0-rc5 was released April 16, 2026. Key changes in v3.4:

• parentUIDToChildren switched to map-of-sets — significant memory and CPU improvement for large Application counts
• ApplicationSet improvements (SOPS Age integration documented in KSail via docs: add SOPS Age integration to ArgoCD ApplicationSet guide and cross-links #4169)

When stable ships, KSail's embedded ArgoCD client (go.mod) should be updated.

Acceptance Criteria

• When ArgoCD v3.4.0 stable is released, update go.mod to embed the stable client
• Verify ksail cluster create --gitops-engine ArgoCD works end-to-end with v3.4
• Run existing ArgoCD system tests; fix any regressions
• Update ArgoCD Helm chart version in the ArgoCD installer if needed
• Note any v3.4 API breaking changes in the changelog

Notes

• Complexity: Low — update + compat test; no API changes expected
• Monitor: https://github.com/argoproj/argo-cd/releases
• Precedent: chore: update ArgoCD Helm chart to 9.5.2 and app image to v3.3.7 #4166 (ArgoCD Helm chart 9.5.2 / app v3.3.7)
• Related: Weekly Strategy #4198

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