[codex] Update GitHub release notes template

[Widthdom]

Summary

• Change tools/CodeIndex.Changelog -- release-notes to emit the requested GitHub release-note template: What's Changed, a full changelog compare link, and Homebrew/NuGet install or update commands.
• Resolve the previous release version from GitHub Releases in the release workflow, excluding draft/prerelease releases and the target tag itself for idempotent reruns, then pass it to release-notes --previous-version.
• Stop appending separate install notes to curated release notes in the release workflow to avoid duplicate install sections.
• Increase the shared CI test session timeout to 45 minutes after Windows net9 timed out at the 30-minute session limit despite reporting zero failed tests.

Validation

• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release --filter "FullyQualifiedName~CodeIndex.Tests.ChangelogToolTests|FullyQualifiedName~CodeIndex.Tests.ReleaseWorkflowTests" -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- release-notes --version 1.28.5 --previous-version 1.28.4
• dotnet run --project tools/CodeIndex.Changelog -- release-notes --version 1.28.5 (expected failure: missing --previous-version)
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release --filter "FullyQualifiedName~CodeIndex.Tests.CiWorkflowTests" -p:UseSharedCompilation=false
• dotnet build src/CodeIndex/CodeIndex.csproj -c Debug -p:UseSharedCompilation=false
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json

Documentation and changelog

• Updated .codex/workflows/release-changelog.md to document the new release-note template and GitHub Release-derived previous version.
• Updated TESTING_GUIDE.md to document the 45-minute CI test session timeout.
• Added/updated changelog.d/unreleased/+release-notes-template.changed.md.
• Added changelog.d/unreleased/+ci-test-session-timeout.internal.md.

No issue auto-close references: this request was not tied to a GitHub issue.