docs: trim and update .github/copilot-instructions.md

[sajayantony]

Summary

Rewrite .github/copilot-instructions.md to be more concise and add guidance learned from #346 / #348 (Go-compatible JSON serialization fix).

This was originally part of PR #348 but was split out per reviewer feedback (one logical change per PR).

Motivation

The current instructions file (130 lines, ~8.7 KB) has:

• Verbose prose that can be compressed without losing signal
• Duplicate sections (Go→.NET table repeats translation rules from Spec & Lineage)
• Missing guidance for serialization (OciJsonSerializer is now the required path)
• Missing testing conventions discovered during fix: use Go-compatible JSON encoding for OCI manifests #348 (raw string JSON fixtures, round-trip testing, internal method parameter rules)
• Sections with low value for Copilot (Copilot Prompt Hints, Non-Goals, trailing --- footer)

Proposed Changes

Trim from ~130 to ~100 lines (−28%) with these specific edits:

Remove

• Go → .NET Guideline Quick Map table — duplicates bullets already in Spec & Lineage
• Copilot Prompt Hints section — low signal
• Non-Goals section — already implied by other rules
• Public API Change Gate — mostly duplicates PR Guidance + Coding Style
• Trailing --- footer line

Add

• Serialization section:
  • All JSON must go through OciJsonSerializer (OrasProject.Oras.Serialization)
  • No direct JsonSerializer calls
  • Ensures Go-compatible encoding (+ not \u002B)
  • Serialization types are internal; domain types have no serialization logic
• Testing additions:
  • Raw string literals for JSON fixtures; wire format over object factories
  • Round-trip: test serialize → deserialize and reverse through same path
• Coding Style additions:
  • Spaces not tabs reminder (editorconfig + CI super-linter enforces)
  • No dead code — remove unreachable branches; verify with coverage
  • Name tuple elements in return types (e.g., (Descriptor Descriptor, byte[] Content))
  • Line length: clarify URLs may exceed the 120 hard limit

Condense (same content, fewer words)

• Section headers shortened (e.g., Project Snapshot → Project)
• Bullet points tightened throughout
• Go→C# translation rules nested under Spec & Lineage as sub-bullets

Review Notes

• The Internal methods: no default parameters (including CancellationToken); prefer overloads rule was flagged in fix: use Go-compatible JSON encoding for OCI manifests #348 review as potentially contradicting existing codebase patterns where internal methods do default CancellationToken. This should be discussed and resolved in the PR for this issue.
• The proposed rewrite was tested as commit 4e6176f on branch agent/fix-plus-escaping-346 before being reverted from fix: use Go-compatible JSON encoding for OCI manifests #348.

References

• PR fix: use Go-compatible JSON encoding for OCI manifests #348 (review thread requesting split): fix: use Go-compatible JSON encoding for OCI manifests #348
• Issue Packer.PackManifestAsync escapes '+' in media types as \\u002B #346 (original serialization bug): Packer.PackManifestAsync escapes '+' in media types as \\u002B #346