Fix unsupported JSON help for mcp and completions

[Widthdom]

Summary

• Document that mcp and completions do not support --json in subcommand help notes.
• Make direct --json errors for both commands explain the command-specific output format.
• Add focused CLI coverage for help and error paths.

Validation

• dotnet format CodeIndex.sln --verify-no-changes --no-restore --verbosity minimal
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --framework net8.0 --filter "FullyQualifiedName~ProgramCliTests"
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --framework net9.0 --filter "FullyQualifiedName~ProgramCliTests"
• CLI smoke checks for mcp --help, completions --help, mcp --json, and completions --json
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json

Documentation / Changelog

• Added changelog.d/unreleased/1424.fixed.md.
• No AGENT.md, CLAUDE.md, or AGENT_GUIDE.md changes.

Review

• Required Codex adversarial review completed with no blocking/actionable issues found.

Follow-up Candidates

• None for this PR.

Fixes #1424