ci: add copilot-setup-steps workflow for coding-agent parity

[devantler]

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What

Adds .github/workflows/copilot-setup-steps.yml so the GitHub Copilot coding agent gets a working .NET toolchain provisioned before it starts — bringing dotnet-template to parity with the sibling templates that already ship this file (go-template, platform-template).

Why

Template-suite parity is the lone open theme of the roadmap epic #218 (themes 1 & 2 — onboarding script + toolchain-floor policy — are already delivered on main). A scan of the shared workflow surface across the four templates found copilot-setup-steps.yml present in go-template and platform-template but absent from dotnet-template (and gitops-tenant-template). Templates are adopter-facing infrastructure: anyone who forks dotnet-template and uses the Copilot coding agent currently gets an un-provisioned environment, whereas a go-template/platform-template fork does not.

Design

• Hardened pattern (reconcile toward the best sibling): mirrors platform-template's structure — top-level permissions: {}, job-scoped contents: read, step-security/harden-runner (egress audit), actions/checkout with persist-credentials: false.
• SDK from global.json: actions/setup-dotnet with global-json-file: global.json so the agent toolchain tracks the repo's single-source SDK floor and never drifts — the .NET parity to go-template's go-version-file: go.mod.
• Self-validating: the workflow triggers on push/pull_request to its own path (plus workflow_dispatch), so this PR's own run exercises the setup end-to-end (dotnet restore Example.slnx).
• All action SHAs pinned to the org-canonical versions used in reusable-workflows (harden-runner v2.19.4, checkout v6.0.3, setup-dotnet v5.3.0).

Validation

• actionlint — clean (exit 0, no findings).
• dotnet restore Example.slnx locally on the repo's global.json floor (SDK 10.0.300, latestFeature) — restores both projects cleanly.
• The PR's copilot-setup-steps check validates it on a GitHub runner.

Note for the maintainer

You use Claude Code, not the Copilot coding agent — but this file is adopter-facing (it benefits people who fork the template), which is presumably why it already lives in two of the four templates. If you're instead phasing copilot-setup-steps out of the suite, the cleaner direction would be to remove it from go-template/platform-template too — happy to do that instead; just close this and say so.

Part of #218 (template-suite parity).