Document changelog validation and release notes flow

[Widthdom]

Summary

• Documented the changelog fragment validator in the fragment, release, README, and precommit workflows.
• Added a release-notes command to tools/CodeIndex.Changelog and wired the release workflow to publish curated CHANGELOG.md notes by default.
• Kept GitHub generated release notes available only as an explicit workflow_dispatch fallback.

Validation

• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet run --project tools/CodeIndex.Changelog -- release-notes --version 1.25.1
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release --filter "FullyQualifiedName~ChangelogToolTests" -p:UseSharedCompilation=false
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• Attempted full dotnet test CodeIndex.sln -c Release -p:UseSharedCompilation=false --no-build; it produced one transient unrelated QueryCommandRunnerTests failure, the failed test passed when rerun directly, and the long-running full process was stopped.

Documentation / Changelog

• changelog.d/unreleased/1999.docs.md
• changelog.d/unreleased/1849.fixed.md
• Updated .codex/workflows/changelog-fragment.md, .codex/workflows/precommit.md, .codex/workflows/release-changelog.md, and changelog.d/README.md.

Follow-up candidates

• None.

Fixes #1999
Fixes #1849