Fix HEAD-scoped index freshness

[Widthdom]

Summary

• Resolve commit-ish refs such as HEAD when stamping commit-scoped freshness for --commits.
• Treat --changed-between <old-ref> HEAD as covering the current HEAD for status --check without advancing the full-scan-only indexed_head_commit.
• Pre-validate invalid --commits refs before DB/exclude setup, and update help text, agent guidance, tests, and the changelog fragment.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~IndexCommandRunnerTests.Run_CommitsInvalidCommitRef_JsonRejectsBeforeDbSetup|FullyQualifiedName~IndexCommandRunnerTests.RunStatusCheck_AfterChangedBetweenRefreshAtHead_DotCommandRestampsFullScanHead|FullyQualifiedName~IndexCommandRunnerTests.RunStatusCheck_AfterCommitScopedRefreshAtHead_DoesNotReportHeadChanged|FullyQualifiedName~IndexCommandRunnerTests.Run_UpdateMode_DoesNotOverwriteIndexedHeadCommit|FullyQualifiedName~IndexCommandRunnerTests.RunStatusCheck_AfterFilesRefreshAtHead_StillReportsHeadChanged|FullyQualifiedName~IndexCommandRunnerTests.ParseArgs_CommitsAcceptsCommitishRefsForGitValidation|FullyQualifiedName~IndexCommandRunnerTests.Run_RebuildWithCommits_JsonIncludesHint|FullyQualifiedName~ConsoleUiTests|FullyQualifiedName~ProgramCliTests.HelpAll_ContainsFullCommandReference"
• dotnet build
• dotnet format CodeIndex.sln --verify-no-changes
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Codex adversarial review follow-up: No blocking/actionable issues found.

Follow-up candidates

None.

Fixes #2809