[CaffeinatedCoder]

Closes #346

The 10.x half of #346: a git cherry-pick -x of #347 commit (a86ae01), which has green CI on main. The closing keyword is here only because pr-hygiene.sh requires one on every PR, as on #339. Against release/10.x it closes nothing; #347 closes the issue on main.

Problem

PackageReleaseNotes is a link to blob/main/src/<package>/CHANGELOG.md. nuget.org's Release Notes tab and Visual Studio's Updates tab show that field, frozen per version, so a 10.x package sends its reader to main's changelog of the day instead of the notes for that version.

Change

The same change as #347. The rendering measurements, the design and the full seed table are there.

• Directory.Build.targets (new): before GenerateNuspec, pack writes the ## [$(Version)] section of the package's own CHANGELOG.md as release notes. nuget.org shows release notes as plain text and links only bare URLs (measured, see chore: write each package's changelog section into its NuGet release notes #347), so the notes are written in plain text: [text](url) becomes text (url), and backticks, ** and ### are dropped. A footer line links blob/v$(Version)/src/<package>/CHANGELOG.md at the version's immutable tag. Pack fails, naming the file, when the section is missing, empty or doubled, or the notes exceed 35,000 characters.
• Directory.Build.props: the link is removed.
• PackagingConventionTests: the link test is replaced by EveryPackage_ShipsItsVersionsChangelogSectionAsReleaseNotes, which generates each nuspec through NuGet's pack task and compares it with the section, and by Pack_FailsNamingTheChangelog_WhenTheSectionCannotBeTheReleaseNotes (missing, empty, doubled, too long).
• test/consumer-smoke-test.sh: the script reads each .nupkg's release notes, which must hold the section's first line, the tag footer and no Markdown.
• AGENTS.md, CONTRIBUTING.md, docs/testing.md: as on main.

Conflicts resolved:

• AGENTS.md: kept 10.x's text (no 11-line notes, the SMOKE_AOT_SDK and test/Directory.Build.targets bullets) and applied the two release-notes sentences to it.
• test/consumer-smoke-test.sh: 10.x has no finish() helper, so the block was added without it.

No changelog line on this line. release/10.x's Directory.Build.props is at 10.1.0, which is tagged and released, and no next 10.x version or milestone has been opened. A line under the released ## [10.1.0] would describe a package that does not have it. The line goes into the next 10.x section when that version is opened (## [10.2.0], or whatever is opened). Until then, a pack here writes 10.1.0's section, which is what 10.1.0 would have shipped. No milestone, for the same reason: the 10.1.0 milestone is a released version.

Tests

Locally on SDK 10.0.303, unit and convention suites as CI runs them: 2,531 passed, 0 failed (Conventions 410, core 1,397, EF 303, NodaTime 238, AspNetCore 92, EF NodaTime 91).

Seeds re-run on this branch (apply, run PackagingConventionTests, git reset --hard):

• The target reading 10.0.0's section instead of $(Version)'s fails EveryPackage_ShipsItsVersionsChangelogSectionAsReleaseNotes and the four Pack_Fails… rows.
• A missing section that no longer fails the pack fails Pack_Fails…("missing").

The other six seeds (the old link wiring, a missing changelog section, kept backticks, the dropped length limit, a blob/main footer, AfterTargets) are in #347 and run against identical code.

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