Update package READMEs as part of the release process

[Copilot]

The release skills and process docs had no step to keep the shared NuGet embedded README (src/PACKAGE.md) current -- no package-list closure enforcement, no badge strategy, no release-notes link. The existing snippet-validation reference also pointed to per-package README paths that don't exist.

Reference docs

• New prepare-release/references/readme-content.md -- maintenance checklist covering: single-shared-file invariant, package-list closure (non-counting phrasing), badge strategy (nuget/vpre during prerelease / nuget/v for stable), release-notes link pattern, and root README.md sync.
• Fixed prepare-release/references/readme-snippets.md -- replaced stale "Which READMEs to Validate" table (src/ModelContextProtocol.Core/README.md, src/ModelContextProtocol.AspNetCore/README.md -- neither exists) with the actual src/PACKAGE.md.

Skill updates

• prepare-release Step 9 -- expanded to run both the content checklist (closure, badges, release-notes link, root sync) and snippet validation. Added edge cases: new package introduced, release type change, forward-referencing release-notes link.
• publish-release Step 5 -- broadened from snippet-only to a full README review pass with final suggestions before the release is created. Added edge cases: stale closure, wrong badge style, missing/incorrect release-notes link.

Process docs

• release-process.md -- notes the README update work in both the prepare and publish sections.

Live illustrative fix to src/PACKAGE.md

This change does not affect the already-published 2.0.0-preview.1 package. It illustrates the README updates that will apply for 2.0.0-preview.2+.

• Added ModelContextProtocol.Extensions.Apps to the package list (was omitted, including from its own embedded README on nuget.org).
• Rewrote "This SDK consists of three main packages" to "The SDK packages are:" -- avoids a stale count when packages are added.
• Switched all four package badges from nuget/v to nuget/vpre -- nuget/v renders nothing during a prerelease-only series.
• Added a release-notes link to https://github.com/modelcontextprotocol/csharp-sdk/releases/tag/v2.0.0-preview.1.

Root README.md mirrors the package-list closure change (Extensions.Apps added, non-counting phrasing); its badges remain nuget/v since that view is GitHub, not nuget.org.

Original prompt

Task: Keep NuGet embedded package READMEs current as part of the release process

Goal

The release process (.github/release-process.md) and the prepare-release /
publish-release skills are missing a step to refresh the NuGet embedded
README for each release. Add that step to the skills + process docs, fix a stale
reference doc, and apply an illustrative fix to the live shared README.

Make all changes on a new branch and open a pull request against main titled
Update package READMEs as part of the release process.

Essential codebase facts (verify before editing)

• All four packages share a single embedded README: src/PACKAGE.md. Each
  csproj packs it via <None Include="..\PACKAGE.md" Pack="true" PackagePath="\README.md" />
  with <PackageReadmeFile>README.md</PackageReadmeFile>. Updating src/PACKAGE.md
  updates every package's nuget.org README at once. The four packages are:
  ModelContextProtocol.Core, ModelContextProtocol,
  ModelContextProtocol.AspNetCore, and ModelContextProtocol.Extensions.Apps.
• src/PACKAGE.md currently says "This SDK consists of three main packages" and
  lists only Core / ModelContextProtocol / AspNetCore — it omits
  ModelContextProtocol.Extensions.Apps (including not listing itself in its own
  embedded README on nuget.org).
• Badges in src/PACKAGE.md use shields.io nuget/v/... which always render the
  latest stable version (currently 1.4.0), not the prerelease series the current
  packages belong to.
• The root README.md (the GitHub repo readme; NOT packed into packages) carries
  the same "three main packages" closure and should be kept in sync.
• .github/skills/prepare-release/references/readme-snippets.md is stale: its
  "Which READMEs to Validate" table lists src/ModelContextProtocol.Core/README.md
  and src/ModelContextProtocol.AspNetCore/README.md, which DO NOT EXIST. The only
  packed README is src/PACKAGE.md.
• Current version in src/Directory.Build.props: <VersionPrefix>2.0.0</VersionPrefix>
  • <VersionSuffix>preview.1</VersionSuffix> → 2.0.0-preview.1.

Confirmed decisions

• Badge strategy: during a prerelease series use shields.io nuget/vpre badges
  (latest including prereleases); for stable releases use nuget/v. Document both.
• Release-notes link: add ONE statement to the shared src/PACKAGE.md linking to
  https://github.com/modelcontextprotocol/csharp-sdk/releases/tag/v{version} (the
  same link for all packages, since the README is shared). The tag will not exist at
  prepare time but resolves by publish time.
• Fix the live src/PACKAGE.md now, illustratively. The PR description MUST state
  that this change does NOT affect the already-published 2.0.0-preview.1 package; it
  illustrates what the README updates will look like for 2.0.0-preview.2+.
• Correct the stale readme-snippets.md reference to target src/PACKAGE.md.

Changes to make

1. New reference: .github/skills/prepare-release/references/readme-content.md

Create a maintenance checklist for the shared embedded README covering:

• The single-shared-src/PACKAGE.md invariant (one file → every package README).
• Package-list closure: every shipping SDK package must be listed (including any
  newly introduced package, and including the package in its own README). Reword the
  "three main packages" phrasing into a non-counting closure so adding a package
  doesn't leave a stale count.
• Badge strategy: nuget/vpre during a prerelease series, nuget/v for stable;
  apply consistently to all package badges.
• Release-notes link: a statement linking to
  https://github.com/modelcontextprotocol/csharp-sdk/releases/tag/v{version}.
• Root README.md sync: keep the repo readme's package-list closure aligned.
• Other salient new data: new package descriptions, getting-started pointers, etc.

2. Fix .github/skills/prepare-release/references/readme-snippets.md

Replace the stale "Which READMEs to Validate" table (Core/AspNetCore per-package
READMEs that do not exist) with the actual packed file src/PACKAGE.md. Update the
"What to Extract" / source-file guidance so snippet validation extracts from
src/PACKAGE.md (and the root README.md if relevant). Keep the rest of the
validation procedure intact.

3. Update .github/skills/prepare-release/SKILL.md

In Step 9 ("Review and Update Documentation"), sub-step 1 ("NuGet package READMEs"):
require BOTH (a) the existing snippet validation AND (b) running the README-content
checklist from the new references/readme-content.md — verify package closure, apply
the prerelease/stable badge strategy, and add/update the release-notes link for
v{version}. Reference the new doc. Add Edge Cases entries, e.g.:

• "New package introduced → add it to the README package closure."
• "Release tag does not yet exist at prepare time → th...