Tag-time branch steps assume a net11 branch that was never created #280

Problem

origin has only main and release/9.x. No net11 branch has ever existed, and no v11.0.0-preview.* tag either: #103 (preview.1) and #157 are still open under #147. These places describe net11 as if it existed:

• AGENTS.md (Branches) and CONTRIBUTING.md "Until the v10.0.0 tag": "net11 holds the .NET 11 retarget … ships 11.0.0-preview.N"
• CONTRIBUTING.md "At the v10.0.0 tag": "Cut release/10.x from the tag, merge net11 into main, and delete net11"
• .github/workflows/dotnet.yml:7-8
• SECURITY.md:59, 65-66, 77: "The 11.0.0 previews continue"
• Epic: 10.0.0 #58's "Done when": "net11 has merged the tag forward (Two supported lines: policy, branch rules, SECURITY rows, and the silent-fix rule as a minor with a changed-answers section #151)"
• Epic: 11.0.0 #147's sequence, step 1

So the tag-time procedure can't be followed as written: only "cut release/10.x" is possible. After the cut, main is "the 11 line" but still says Version 10.0.0, so the retarget (#103) has to land on main directly.

Change (needs a maintainer decision)

Either:

• (a) say that the previews start on main after the tag: drop net11 from the docs, Epic: 10.0.0 #58 and Epic: 11.0.0 #147, and have 11.0.0-preview.1: retarget main to net11.0 / EF Core 11 / Npgsql 11 after the v10.0.0 tag, with the four behaviour changes as named tests #103 retarget main right after release/10.x is cut; or
• (b) create net11 before the tag, as documented.

(a) matches what actually happened. It also makes #157 moot, because 10.0.0 will be on NuGet before the first preview.

Done when

AGENTS.md, CONTRIBUTING.md, SECURITY.md, dotnet.yml, #58 and #147 describe the same procedure, and it can be followed at the tag.