ci: use shared helm-charts-validation reusable workflow

[rguichard]

Replaces the local helm-charts-validation.yaml with a thin caller of the shared reusable morpho-org/github-actions/.github/workflows/helm-charts-validation.yml@v1.

Iso preserved: push+PR on morpho-main (paths helm/**, plus the workflow file so it self-validates), runner ubuntu-latest, chart-paths helm/charts helm/environments (default), skip-patterns *-db,*database*,*postgres*.

Part of PLA-1787
Implements PLA-1870

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[rguichard]

⚠️ Blocked — needs erpc Actions config investigation (UI).

The reusable-calling workflow startup-fails ("This run likely failed because of a workflow file issue", 0 jobs, no step logs) — the same symptom as erpc's wiz-iac-scan migration attempt. Diagnostics so far:

• morpho-org/github-actions access level = organization (erpc is allowed to call it).
• morpho-api and morpho-infra-helm consume the exact same reusable successfully; morpho-api has identical Actions settings to erpc (allowed_actions: selected, same allowlist, sha_pinning_required: true).
• Pinning the reusable to a full commit SHA (instead of @v1) did not help → not a sha-pinning issue.

So this is an erpc-specific blocker not visible via the API. The exact startup error is on the run page: https://github.com/morpho-org/erpc/actions/runs/29335970576 — please check erpc's Actions settings / org rulesets there. Leaving this PR open until erpc can call shared reusables. Tracked under PLA-1870 / PLA-1787.