Add Copilot PR review instructions for modern Go (1.26) patterns

[kristenwomack]

Problem

Copilot PR reviewer flagged 5 false positives on PR #7223 because it didn't know about Go 1.26 language features:

• 4 comments claimed new(false)/new(true) is invalid Go — Go 1.26 supports new(expr) for pointer literals
• 1 comment claimed internal.ErrorWithSuggestion had a missing import — the import existed outside the diff context

These patterns are already documented in cli/azd/AGENTS.md (lines 185-197, 'Modern Go Patterns'), but the Copilot PR reviewer reads .github/instructions/*.instructions.md files, not AGENTS.md.

Proposed Fix

Create .github/instructions/go.instructions.md with applyTo: '**/*.go' containing the modern Go 1.26 patterns most likely to trigger false positives, plus guidance to check full file context before flagging missing imports.

Context

• PR with false positives: functionapp: handle .funcignore more gracefully #7223
• Reviewer feedback: functionapp: handle .funcignore more gracefully #7223 (comment)
• AGENTS.md modern Go section: cli/azd/AGENTS.md lines 176-197