[AIAA] Add Copilot cloud agent environment setup workflow

[nate-double-u]

Adds .github/workflows/copilot-setup-steps.yml: prepares the Copilot
cloud agent's ephemeral environment. Build step 4 (spec section 16),
environment per section 11. Tracking: #372.

What it does

Before the agent's model starts: checkout, Node from .nvmrc with npm
cache, npm ci, via the repository's existing
npm-ci-via-cached-nvmrc composite action. The agent starts with the
toolchain ready to run npm run check instead of discovering it.

Minimal on purpose: the deterministic data-collection scripts join
these setup steps in a later build step, once they exist.

Guardrails

• Setup runs outside the agent's firewall, so this reviewed file is
  the control over what setup fetches (section 11). It fetches only
  the repository and npm dependencies.
• No secrets introduced; permissions: contents: read only
  (section 11 secrets policy).
• Single job named copilot-setup-steps, the platform's contract;
  timeout-minutes: 10, well under the 59-minute session cap.

Verification

• Assertion script (session artifact): 15 checks pass (job name,
  self-scoped validation triggers, least permissions, exact steps, no
  secrets).
• npm run check passes.
• The workflow triggers on changes to itself, so this PR shows a green
  "Copilot Setup Steps" run before merge.
• Post-merge: an agent session's logs showing the prepared environment
  is the done-when; lands with the probe delegation ([AIAA] Build probe: first cloud-agent delegation (section 17 checks 1-5) #366) or the
  first delegated session, recorded on [AIAA] Build step 4: cloud agent environment setup #372.

Contributes to: #372

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[Copilot]

Pull request overview

Adds the Copilot cloud agent setup workflow, preparing Node.js and npm dependencies before agent execution.

Changes:

• Adds self-validation and manual triggers.
• Uses read-only permissions and a 10-minute timeout.
• Reuses the existing cached npm installation action.

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