Document set-and-forget fleet expansion

[Nickfost]

Closes #18.

Summary

• define the invariant that host and worker configuration never contains repository names
• add a repeatable runbook for enrolling Docker hosts at any site
• add a project-onboarding runbook that requires no host changes
• document unique per-host scale sets, shared capability labels, runner-group policies, maintenance, backup, draining, replacement, and removal
• link the new how-tos from the README, architecture, and agent rules

Operational result

A compatible host is configured once. Future repositories change only project code, private policy, and GitHub runner-group access. Future locations join the same queue with a unique instance/scale-set name and the shared routing label.

Verification

• compared branch to main: five commits, five intended documentation files, no unrelated changes
• all links use repository-relative targets or existing official documentation
• no credentials, internal inventory, addresses, or organization-private values added