[experimental] Add experimental/agent_compose placeholder with preview pointer

experimental/agent_compose/README.md

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+# Agent Compose (experimental)
+
+Agent Compose is an experimental effort to make Megatron-LM development
+agentic-native: composing Megatron Core primitives with coding agents, rather
+than introducing a new standalone product or training stack.
+
+This directory is a placeholder that establishes the location and naming for
+the upstreamed work. Content will land here incrementally as a series of small,
+reviewable PRs.
+
+## Preview
+
+The full work-in-progress implementation lives on the `dev` branch under
+`experimental/lite/`:
+
+- https://github.com/NVIDIA/Megatron-LM/tree/dev/experimental/lite
+
+The preview currently includes:
+
+- A lightweight runtime API built from small composable primitives.
+- Native model implementations with explicit model/runtime protocols.
+- Hugging Face safetensors load/export helpers.
+- Validation recipes and benchmark examples against Megatron-Core reference
+  paths (bitwise loss/grad-norm parity on the distributed-optimizer path).
+- Skills playbooks that let coding agents extend models and primitives in a
+  reviewable way.
+
+## Principles
+
+- **Compose, don't fork.** Primitives reuse and build from existing Megatron
+  Core modules wherever appropriate. When a primitive cannot reuse an existing
+  module and needs a separate implementation, the reason is documented in the
+  docstring, making gaps explicit and providing input for future Megatron Core
+  improvements.
+- **Reviewable by construction.** Runtime, model, and primitive code are split
+  into small contracts so agents and humans can make targeted changes without
+  touching unrelated Megatron subsystems.
+- **Core performance.** Changes are validated against Megatron-Core reference
+  paths for both correctness and speed.
+
+## Status
+
+Upstreaming is being scoped: the current work is being evaluated for splitting
+into small PRs, after which a timeline will be shared. Until then, please use
+the preview branch above.