feat: prepare internal testnet release with tagged builds, agent monitoring, and onboarding improvements

[matthias-wright]

Changes:

• Pin Summit, seismic-reth, and Custodian builds to internal-testnet-v0, with safe tag checkout and rerun support.
• Add optional Prometheus Agent installation for validators and observers, including node lifecycle integration and monitoring commands.
• Support lifecycle-gated validator onboarding without checkpoints.
• Derive onboarding identity from installed keys; make deposit-signature input optional with identity consistency checks.
• Update internal-testnet genesis fields for the upgraded stack.