Fix subcommand help usage

[Widthdom]

Summary

• Print command-specific usage for cdidx <command> --help instead of the full top-level help.
• Add completions as a command alias for shell completion generation while preserving the legacy --completions behavior.
• Cover command-specific help and alias-specific completion errors in CLI regression tests.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~ProgramCliTests.SubcommandHelp_PrintsCommandSpecificUsage|FullyQualifiedName~ProgramCliTests.CompletionsCommand_ErrorsUseCommandUsage|FullyQualifiedName~ProgramCliTests.Completions_HelpLikeValueReturnsCompletionsError|FullyQualifiedName~CliFlagSchemaTests" -p:UseSharedCompilation=false
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• Smoke checks: cdidx search --help, cdidx index --help, cdidx license --help, cdidx completions --help, cdidx completions --json

Documentation and Changelog

• Added changelog fragment changelog.d/unreleased/1693.fixed.md.
• No docs changes were required because this only changes CLI help output discoverability.

Review

• Adversarial review found actionable issues for license --help and completions alias usage errors; both were fixed.
• A follow-up review found the branch was behind origin/main and showed unrelated reverted changes in the diff; latest origin/main was merged until origin/main..HEAD only contained the Per-subcommand --help does not print contextual usage for any subcommand #1693 files.

Fixes #1693