Add comprehensive AGENTS.md implementation reference

[Copilot]

Creates AGENTS.md at the repo root as a dense implementation reference for agents (and contributors) working in the codebase.

Coverage

• Repository layout – project roles table (StructId, StructId.Analyzer, StructId.CodeFix, tests, packaging)
• Core interfaces – IStructId, IStructId<TValue>, INewable<TSelf[,TValue]>, TStructIdAttribute, TValueAttribute
• Generator architecture – BaseGenerator pipeline and ReferenceCheck modes (ValueIsType vs TypeExists); TemplatedGenerator discovery/expansion flow; per-generator table with reference types, template files, and activation conditions
• Template system – CodeTemplate.Apply mechanics, TemplateRewriter/ValueRewriter substitution rules (TSelf, TValue, prefixed variants), namespace injection, [TStructId] authoring rules and constraint patterns, [TValue] templates and TemplatizedTValueExtensions
• Cacheable models – why StructIdModel/TemplateModel/TValueTemplateModel exist (no ISymbol/Compilation in pipeline), EquatableArray<T>, TrackingNames
• Diagnostics & code fixes – SID001–SID005 with triggers and associated fix providers
• Testing patterns – StructIdGeneratorTest<TGenerator>, ReferenceAssemblies.Net.Net80, incrementality test conventions
• Key design decisions – why readonly record struct, file-scoped templates, dev-only package, Scriban vs CodeTemplate (Dapper), ReferenceCheck semantics
• New integration guide – step-by-step for adding a generator

[kzu]

🧪 Details on Ubuntu 24.04.4 LTS

🧪 Details on macOS Unix 15.7.4

🧪 Details on Microsoft Windows 10.0.26100

from retest v1.1.0 on .NET 10.0.5 with 💜 by @devlooped