Add Copilot code review for v2 shim PRs

[shreyanshjain7174]

Add Copilot Code Review for V2 Shim PRs

Adds automated GitHub Copilot code review that triggers only on PRs touching v2 shim code paths.

What

Three files:

File	Purpose
.github/copilot-instructions.md	Global review tone: concise, prefixed ([BLOCKER]/[ISSUE]/[SUGGESTION]/[QUESTION]), no speculation
.github/instructions/v2-shim.instructions.md	Go best practices + hcsshim-specific review rules
.github/workflows/copilot-review.yml	Auto-request Copilot review when v2 shim paths change

Why

V2 shim code has specific patterns that are easy to get wrong:

• Resource lifecycle (ResourceCloser registration, Close() ordering, deferred cleanup)
• Handle/memory leaks (Windows handles, UVM device mounts)
• Concurrency (hcs.System is not goroutine-safe, UVM device maps are mutex-protected)
• Package layering violations

Having Copilot flag these before human review saves reviewer time.

Design

• Copilot = guidance only (non-blocking). CI remains the enforcement gate.
• Path-scoped: only fires when shim binaries, internal/hcs*, internal/uvm, internal/controller, internal/vm, etc. are touched.
• Go-first: the instruction file leads with standard Go conventions (Effective Go, naming, error handling, exported-vs-unexported, concurrency) and extends with hcsshim-specific rules.
• No model preference hardcoded — model selection is done in GitHub UI settings.
• Skips draft PRs. Idempotent (won't re-request if already requested).

Admin steps required after merge

1. Enable Copilot Code Review on the repo (Settings > Copilot)
2. Select preferred model in Settings > Copilot > Code review
3. Create copilot-review label (or remove label step from workflow)