Open upstream Obol integration PR (overlay + docs, opt-in)

[swarna1101]

Open the integration PR to the confirmed Obol repo once the overlay is validated and Obol has agreed on direction.

Part of #23.

Scope

• Contribute compose-optimum.yml overlay + docs entry, opt-in and off by default.
• Docs cover: enabling via COMPOSE_FILE, API-key prerequisite, supported CL clients, and how to disable.
• Address Obol review feedback.

Depends on

• Overlay built
• Hoodi validation passed
• Target repo confirmed + Obol aligned

Done when

• PR opened against the confirmed upstream repo, opt-in/off by default, docs included.