[experimental] Add experimental/agent_compose placeholder with preview pointer

[ISEEKYAN]

This PR establishes experimental/agent_compose/ on main as the landing
location for the agent-compose work (composing Megatron Core primitives with
coding agents — see the recent naming/fork-cleanup discussion).

It contains only a README that:

• states the intent and principles (compose/reuse Megatron Core modules,
  document any gaps in docstrings, reviewable-by-construction),
• points to the work-in-progress preview at
  https://github.com/NVIDIA/Megatron-LM/tree/dev/experimental/lite,
• notes that content will land incrementally as small, reviewable PRs once
  the splitting effort has been scoped.

No code changes; no impact on any existing path.

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