Update release processes to support release servicing branches (#1620)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.github/skills/shared-resources/release-branches.md

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+# Release Branches
+
+Shared reference for release skills. Describes the branch roles used by the release workflow and the rules each skill follows for selecting a branch and looking up the previous release.
+
+## Branch roles
+
+| Branch              | Purpose                                         | CI behavior                            |
+| ------------------- | ----------------------------------------------- | -------------------------------------- |
+| `main`              | Next-MAJOR preview/development line             | Nightly `cron` build → GitHub Packages |
+| `release/{MAJOR}.x` | Long-lived servicing branch for a shipped MAJOR | Every push → GitHub Packages           |
+| `release-{version}` | Short-lived release preparation branch          | Built by PR CI; no package publishing  |
+
+Official NuGet.org publishes happen only when a GitHub Release is created from a branch's tag.
+
+## Selecting a source/base branch (`prepare-release` Step 1)
+
+1. List candidate branches via:
+   `gh api repos/{owner}/{repo}/branches --paginate --jq '[.[] | select(.name == "main" or (.name | startswith("release/"))) | .name]'`
+2. Present the list to the user. Default selection: `main`.
+3. The selected branch drives:
+   - Previous-release lookup (see below).
+   - The branch on which the candidate version is read from `src/Directory.Build.props`.
+   - The commit range from which PRs are collected.
+   - The `--base` of the PR created at the end of the skill.
+
+## Previous-release tag lookup
+
+- On `main`: most recent published release globally (use `gh release list --exclude-drafts --limit 50` and pick the highest semver). No MAJOR filter.
+- On `release/{MAJOR}.x`: most recent published release whose tag matches `v{MAJOR}.*`. Drafts are excluded.
+
+This is purely a baseline-selection rule. It does **not** change the breaking-change policy. See [the versioning docs](https://csharp.sdk.modelcontextprotocol.io/versioning.html) for the policy.
+
+## Work-branch naming
+
+Prepare-release work branches are named `release-{version}` (flat, hyphen-separated):
+- `release-2.0.0-preview.1`
+- `release-1.3.1`
+- `release-2.0.0`
+
+Hyphens in prerelease versions are valid in git branch names.