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

AGENTS.md

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+# AGENTS.md
+
+OpenTelemetry-compatible Zap logger for Sourcegraph (`github.com/sourcegraph/log`). This is a Go library consumed by other Go services.
+
+## Setup
+
+Requires Go as specified in `go.mod` (`go 1.25.9`). Dependencies are managed with Go modules.
+
+```bash
+go mod download
+```
+
+## Build, test, lint
+
+These are the commands used in CI (`.github/workflows/pipeline.yml`):
+
+```bash
+go build -v ./...
+go test -v -race -covermode=atomic ./...
+```
+
+Run a single package's tests, e.g.:
+
+```bash
+go test -race ./logtest/...
+```
+
+Some tests use `github.com/hexops/autogold/v2` for golden-file assertions. Regenerate golden values with:
+
+```bash
+go test ./... -update
+```
+
+## Layout
+
+- Root package `log` — the public API: `Logger`, `Init`, fields (`fields.go`), levels (`levels.go`), sinks (`sinks*.go`). Start at `doc.go` and `logger.go`.
+- `internal/` — implementation details not part of the public API (`encoders`, `globallogger`, `otelfields`, `sinkcores`, `configurable`, `stderr`).
+- `logr/` — adapter for `go-logr/logr`; this is a **separate Go module** with its own `go.mod`.
+- `logtest/` — testing utilities for code that uses this library.
+- `std/` — `log.Logger`/standard-library adapter.
+- `output/`, `hook/` — output formatting and Zap hook helpers.
+- `cmd/example/` — runnable usage example.
+
+## Conventions
+
+- No global logger: loggers are passed in from callers to preserve scope and fields. Do not introduce package-level or compile-time loggers (see `doc.go`).
+- Scopes should be static, CamelCased values, not dynamic identifiers.
+- Behavior is configured via environment variables: `SRC_DEVELOPMENT`, `SRC_LOG_FORMAT` (`json`, `json_gcp`, `condensed`), `SRC_LOG_LEVEL` (`debug`, `info`, `warn`, `error`, `none`), and `SRC_LOG_SCOPE_LEVEL` (see `init.go`).
+- When changing the `logr/` adapter, run and update tests within that module separately.