Add database maintenance commands

[Widthdom]

Summary

• checkpoint writable DB sessions with WAL TRUNCATE after write work
• add cdidx db schema and db prune dry-run/apply output
• document DB maintenance behavior and add changelog fragments

Fixes #1644
Fixes #1646
Fixes #1728

Validation

• dotnet build
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter 'FullyQualifiedName~DbCommandRunnerTests'
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter 'FullyQualifiedNameDatabaseTests.TryCheckpointWalTruncate|FullyQualifiedNameDatabaseTests.Dispose_AfterWriteWork'
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter 'FullyQualifiedNameDbCommandRunnerTests|FullyQualifiedNameDatabaseTests.TryCheckpointWalTruncate|FullyQualifiedName~DatabaseTests.Dispose_AfterWriteWork'
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedNameDbCommandRunnerTests|FullyQualifiedNameDatabaseTests" (adversarial review)
• dotnet run --project tools/CodeIndex.Changelog -- check
• cdidx status --check --json

Note: a full unfiltered local test run was attempted earlier but failed because local git signing prompts for /Users/widthdom/.ssh/id_ed25519_signing in test-created git commits; targeted tests above passed.