Add instructions

.github/copilot-instructions.md

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+# GitHub Copilot Instructions
+
+Single source of truth: read and follow `../AGENTS.md`.
+
+Do not duplicate instructions here. If anything conflicts, `../AGENTS.md` wins.

AGENTS.md

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+# Markdig — Codex Agent Instructions
+
+Markdig is a fast, powerful, CommonMark compliant, extensible Markdown processor for .NET.
+
+Paths/commands below are relative to this directory.
+
+## Orientation
+
+- Workspace: `src/markdig.slnx` (solution/workspace listing the projects below)
+- Library: `src/Markdig/`
+- Tests: `src/Markdig.Tests/` (NUnit; includes generated spec tests)
+- Signed library build: `src/Markdig.Signed/` (strong-named build of `src/Markdig/` sources)
+- Benchmarks: `src/Markdig.Benchmarks/` (BenchmarkDotNet perf comparisons)
+- Fuzzing: `src/Markdig.Fuzzing/` (SharpFuzz/libFuzzer harness for parser/renderers)
+- Web app: `src/Markdig.WebApp/` (ASP.NET Core API for converting Markdown → HTML)
+- Tooling: `src/mdtoc/` (CLI to generate a Markdown TOC from a local file or GitHub URL)
+- Tooling: `src/SpecFileGen/` (regenerates `*.generated.cs` spec tests from `src/Markdig.Tests/**.md`)
+- Tooling: `src/UnicodeNormDApp/` (generates the Unicode→ASCII mapping used by `Markdig.Helpers.CharNormalizer`)
+- Docs to keep in sync with behavior: `readme.md` and the docs under `doc/` (e.g., `doc/**/*.md`)
+
+## Build & Test
+
+```sh
+# from the project root (this folder)
+cd src
+dotnet build -c Release
+dotnet test -c Release
+```
+
+All tests must pass and docs must be updated before submitting.
+
+## Contribution Rules (Do/Don't)
+
+- Keep diffs focused; avoid drive-by refactors/formatting and unnecessary dependencies.
+- Follow existing patterns and naming; prefer clarity over cleverness.
+- New/changed behavior requires tests; bug fix = regression test first, then fix.
+- All public APIs require XML docs (avoid CS1591) and should document thrown exceptions.
+
+## C# Conventions (Project Defaults)
+
+- Naming: `PascalCase` public/types/namespaces, `camelCase` locals/params, `_camelCase` private fields, `I*` interfaces.
+- Style: file-scoped namespaces; `using` outside namespace (`System` first); `var` when the type is obvious.
+- Nullability: enabled — respect annotations; use `ArgumentNullException.ThrowIfNull()`; prefer `is null`/`is not null`; don't suppress warnings without a justification comment.
+- Exceptions: validate inputs early; throw specific exceptions (e.g., `ArgumentException`/`ArgumentNullException`) with meaningful messages.
+- Async: `Async` suffix; no `async void` (except event handlers); use `ConfigureAwait(false)` in library code; consider `ValueTask<T>` on hot paths.
+
+## Performance / AOT / Trimming
+
+- Minimize allocations (`Span<T>`, `stackalloc`, `ArrayPool<T>`, `StringBuilder` in loops).
+- Keep code AOT/trimmer-friendly: avoid reflection; prefer source generators; use `[DynamicallyAccessedMembers]` when reflection is unavoidable.
+- Use `sealed` for non-inheritable classes; prefer `ReadOnlySpan<char>` for parsing.
+
+## API Design
+
+- Follow .NET guidelines; keep APIs small and hard to misuse.
+- Prefer overloads over optional parameters (binary compatibility); consider `Try*` methods alongside throwing versions.
+- Mark APIs `[Obsolete("message", error: false)]` before removal once stable (can be skipped while pre-release).
+
+## Git / Pre-Submit
+
+- Commits: commit after each self-contained logical step; imperative subject, < 72 chars; one logical change per commit; reference issues when relevant; don't delete unrelated local files.
+- Checklist: each self-contained step is committed; build+tests pass; docs updated if behavior changed; public APIs have XML docs; changes covered by unit tests.

CLAUDE.md

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+# Claude Instructions
+
+Single source of truth: read and follow `AGENTS.md`.
+
+Do not duplicate instructions here. If anything conflicts, `AGENTS.md` wins.