Add Copilot instructions and PR-review prompt synced from armada

[Christdej]

Summary

• Adopts the shared Flotilla-SARA Copilot configuration published in equinor/armada.
• Adds .github/copilot-instructions.md with a placeholder repo-specific header and a synced block (system context + condensed PR-review checklist).
• Adds .github/prompts/review-pr.prompt.md — a slash-command prompt for PR review that auto-detects PR vs. branch diff and emits structured Summary / Blocking / Suggestions / Questions output, with explicit cross-repo impact checks.

Source of truth

Both files are SHA-stamped synced copies of artifacts in equinor/armada (.github/copilot/). To refresh after armada changes:

# from an armada checkout
./scripts/sync-copilot.sh <path-to-flotilla-checkout>

The script preserves any hand-edited content outside the <!-- BEGIN SYNCED FROM equinor/armada@<sha> --> ... <!-- END SYNCED --> markers in copilot-instructions.md, so repo-specific guidance can be added above the synced block without being overwritten on the next sync.

Pilot

This is the first repo to adopt the shared config; once we have feedback from real PR reviews here we'll roll it out to the rest of the Flotilla-SARA repos.

Ready for review checklist

• A self-review has been performed
• All commits run individually
• Temporary changes have been removed
• The PR has been tested locally
• A test has been written
  • This change doesn't need a new test (config-only)
• Relevant issues are linked (none)
• Remaining work is documented in issues
  • There is no remaining work from this PR that requires new issues
• The changes do not introduce dead code