Collapse the experimental channel into main; main is the sole prerelease lane (ADR-0008)

[ChrisonSimtian]

What

Retires the experimental branch and its -alpha GitHub Packages lane. main (-preview) becomes the sole continuous prerelease lane. Steady-state long-lived branches are now main + support/* (+ gh-pages); everything else is a tag or short-lived topic branch.

Decision recorded in ADR-0008, which supersedes the channel-ladder section (§2) of ADR-0004 (its 2026-05-30 amendment). Calendar versioning, the production line, [Experimental], review tiers (ADR-0004 §1, §3–§6), the on-demand cut (ADR-0007), and nuget.org opt-in (ADR-0002) are all unchanged.

Why

The dedicated fast lane didn't earn its keep:

• It ran ~17 commits behind main, not ahead — the forward-port discipline it relied on wasn't happening.
• At collapse time its entire source/build delta vs main was empty — the publish feature it nominally carried (IPublish: multi-channel, package-ID-aware publishing #333/feat(publish): multi-channel, package-ID-aware publishing + dogfood the lanes (#333) #339) already existed on main; it was a stale snapshot.
• It cost a second GitHub Packages publisher, a protected branch, an extra workflow, and a forward-port obligation for no realised benefit.

Breaking work now batches on main gated behind [Experimental("FALLOUT0xx")] (or a short-lived topic branch when it can't be gated) — the attribute was always the per-API isolation tool; it now carries the load the branch used to. The production-cut review is the backstop.

Changes

• CI: delete experimental.yml; preview.yml is the sole continuous publisher. Drop experimental from the ubuntu-latest PR-gate (source: Build.CI.GitHubActions.cs, removed the ExperimentalBranch const). Build verified; generated ubuntu-latest.yml regenerated and in sync.
• ADR: add ADR-0008; "superseded in part" banner on ADR-0004; index updated.
• Docs: AGENTS.md, CONTRIBUTING.md, README, branching-and-release.md, release-and-versioning.md, conventions.md, roadmap.md, experimental-apis.md updated to the two-tier (main → release/YYYY) model. The [Experimental] attribute / experimental label are preserved throughout.

Notes

• The repo-wide ChrisonSimtian → Fallout-build org-repoint (~161 files) is deliberately out of scope — it's a separate follow-up so neither diff is muddied.
• CHANGELOG.md is deprecated (release notes come from PR labels), so no entry added.

Verification

• dotnet build build/_build.csproj ✅ (warnings pre-existing)
• CI workflow regeneration: no drift
• git grep experimental sweep: only [Experimental] attribute/label refs + settled-history notes remain