ctags tagfile export should replace output atomically

[Widthdom]

Finding

export ctags writes directly to the requested output path with StreamWriter(..., append: false, ...). A crash, cancellation, or IO failure can leave a partially written tagfile in place.

Evidence

• src/CodeIndex/Cli/ExportImportCommandRunner.cs:262 creates the output directory.
• src/CodeIndex/Cli/ExportImportCommandRunner.cs:265 opens the final output path directly.
• The command then streams all symbol rows to that final path.

Suggested fix

Write to a temporary file in the same directory, flush it, then atomically replace or move it into place. Add tests for failure before replacement.

[Widthdom]

Work started on branch fix-issue2839-2842-2886-2850.