Mesh: define generic Docker cluster bootstrap contract

[snowdamiz]

Parent epic

• Mesh: clustered Docker-on-VM deploy baseline for Hyperpush #3

Outcome

Define the exact env, identity, discovery, and startup contract for generic Docker deployments of clustered Mesh apps.

Acceptance criteria

• Operator-facing bootstrap inputs are explicit
• Cluster formation works without repo-local hacks
• The contract is stable enough for Hyperpush product deployments