Staging collective setup: runbook + oc-opsdevnz 0.2.5 dep bump

Summary

Adds the staging collective launch runbook for OpsDev.nz (ADR-0018), bumps the oc-opsdevnz dependency to v0.2.5, and fixes a stale staging-host slug in the module's AGENTS.md. Documentation and dependency refresh; no functional changes.

What changed

We have set up the collective in staging:

• https://staging.opencollective.com/opsdevnz

Related

• ADR-0018: Launch OpsDev.nz as a fiscally-hosted collective (StartMeUp.NZ repo)
• oc-opsdevnz v0.2.5 on PyPI — the dependency version this MR bumps to
• The staging launch was executed end-to-end against staging.opencollective.com; output of the actual commands is reflected in the runbook, a slightly-more-specific-then-it-should-be doc we will make more generic later.