ADR: Release-branch model & multi-channel CD #277

Sibling of milestone #13. RFC #267.

What

Write the Architecture Decision Record capturing the release-branch + multi-channel CD decision, the alternatives considered, and the reasoning. Lives under docs/adr/ (create the directory if it doesn't exist; this may be the first ADR).

Acceptance criteria

• docs/adr/0001-release-branch-model.md (or numbered per existing convention if there is one) exists.
• Sections: Status, Context, Decision, Consequences, Alternatives considered, References.
• Context explains the trigger: auto-release on every main merge was generating ≈20 patch versions in recent days and Dependabot fatigue across the userbase.
• Decision matches what landed:
  • main = integration trunk, doesn't auto-publish.
  • release/vN = release channel, tag-triggered.
  • Hotfixes via cherry-pick from main.
  • Three GitHub Environments keyed by channel (nuget-org/github-packages/github-releases), with nuget-org gated by approval.
• Alternatives considered captures: keep main-triggered + reduce noise via path filters; release-per-merge to release/vN; manual-only via workflow_dispatch as the permanent shape.
• Consequences notes: more deliberate cadence; cherry-pick mental overhead; old majors stay supportable; the consumer-pain-first principle this validates (see memory note).
• References link RFC RFC: Release-branch model & multi-channel CD (tag-triggered, GitHub Environments) #267, milestone chore(ci): roll-up of #10, #11, #12 into develop #13, and the sibling implementation issues.
• Status: Accepted once the implementation ships; Proposed until then.

Depends on

• Implementation shipped (so the ADR describes reality, not aspiration).

Notes

If docs/adr/ doesn't exist, this is the seed for it. Worth setting the pattern now — future architectural decisions get ADRs filed under the same directory using the MADR format (or whatever Chris prefers).