docs: add missing AGENTS.md (and README where missing)

AGENTS.md

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+# AGENTS.md
+
+Guidance for coding agents working in this repository.
+
+## Overview
+
+`jsonx` is a single-package Go library (`github.com/sourcegraph/jsonx`) providing
+an error-tolerant JSON parser, scanner, formatter, and editor that supports
+trailing commas and comments. It is ported from Visual Studio Code's
+comment-aware JSON APIs.
+
+## Layout
+
+All source lives in the repository root as a flat `package jsonx`:
+
+- `scanner.go` — tokenizer.
+- `parser.go` / `tree.go` — parsing and node tree.
+- `visitor.go` — streaming visitor API.
+- `format.go` — formatting.
+- `edit.go` — applying edits to JSON text.
+- `json.go` — package doc and `go:generate` directive.
+- `json_stringer.go` — generated `String()` methods (do not edit by hand).
+- `*_test.go` — tests for each file.
+
+## Setup
+
+Requires Go (see `go.mod` for the module's Go version; CI builds against `1.x`).
+There are no third-party dependencies.
+
+```bash
+go build ./...
+```
+
+## Test, lint, generate
+
+```bash
+go test ./...        # run tests
+go vet ./...         # vet
+go generate ./...    # regenerate json_stringer.go after changing enum types
+```
+
+`json_stringer.go` is generated via the `//go:generate stringer` directive in
+`json.go`. Regenerate it rather than editing it directly when changing the
+`ParseErrorCode`, `ScanErrorCode`, `SyntaxKind`, or `NodeType` types.
+
+## Conventions
+
+- Standard Go formatting (`gofmt`); format code before committing.
+- Keep changes within the flat root package layout.