Add AGENTS.md

AGENTS.md

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+# AGENTS.md
+
+This file provides guidance to LLM Agents when working with code in this repository.
+
+## Project
+
+PHP OpenTSDB HTTP API Client (`cybercog/opentsdb-client`) — a library for sending time-series metric data to OpenTSDB via its HTTP API. Also compatible with VictoriaMetrics.
+
+## Commands
+
+All commands must be run inside Docker containers via `docker compose run`.
+
+### Install dependencies
+```bash
+docker compose run php85 composer update --prefer-stable --prefer-dist --no-interaction
+```
+
+### Run all tests
+```bash
+docker compose run php85 vendor/bin/phpunit
+```
+
+### Run unit tests only
+```bash
+docker compose run php85 vendor/bin/phpunit --testsuite Unit
+```
+
+### Run integration tests only (requires running OpenTSDB container)
+```bash
+docker compose run php85 vendor/bin/phpunit --testsuite Integration
+```
+
+### Run a single test
+```bash
+docker compose run php85 vendor/bin/phpunit --filter testMethodName
+```
+
+Available PHP services: `php80` through `php85`. The `opentsdb` service runs on port 4242.
+
+## Architecture
+
+The library has three core classes in `src/` under namespace `Cog\OpenTsdbClient`:
+
+- **`DataPoint`** — Immutable value object representing a single metric. Validates all inputs in the constructor (AssertionError on failure). Sanitizes metric/tag names by replacing unsupported characters with `-`. Implements `JsonSerializable`.
+- **`OpenTsdbClient`** — Main client. Takes a PSR-18 `ClientInterface` via constructor injection. Two send methods: `sendDataPointList()` (fire-and-forget) and `sendDataPointListWithDebug()` (returns response details). Maps 12 HTTP status codes to specific error messages.
+- **`SendDataPointListResponse`** — Immutable response object with httpStatusCode, success/failed counts, and errors.
+
+Exception hierarchy: `OpenTsdbExceptionInterface` marker implemented by `OpenTsdbException` (HTTP errors) and `OpenTsdbConnectionException` (network failures).
+
+## Code Conventions
+
+- `declare(strict_types=1)` in all files
+- All classes are `final`
+- PSR-4 autoloading, PSR-18 HTTP client, PSR-7 messages
+- 4-space indentation, LF line endings
+- Tests: `test/Unit/` and `test/Integration/` with separate PSR-4 namespaces
+- Integration tests are skipped in CI via `APP_ENV=ci` check

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Meta Instructions
+
+**CRITICAL**: When someone asks to add project instructions or runs `/init` to initialize the project context, you MUST update `AGENTS.md`, NOT this file. `AGENTS.md` is the single source of truth for ALL agents, including yourself. This file simply imports it.
+
+@AGENTS.md