refactor: extract CLI execution into cliExec module with improved UX

[EhabY]

• Split core/cliUtils into cliUtils (binary file management) and cliExec (command execution: version, speedtest, ping, openAppStatusTerminal)
• Introduce CliEnv interface so callers don't manage shell escaping; each cliExec function resolves its own flags
• Add resolveClientAndWorkspace for shared workspace resolution (sidebar, connected, or picker)
• Use locateBinary (stat-only) in resolveCliEnv before falling back to fetchBinary
• Surface CLI stderr in error messages instead of leaking full binary paths
• Validate Go duration input in speed test prompt
• Rename "Run Speed Test" → "Speed Test Workspace" in command palette, "Speed Test" in sidebar
• Use separate :views command IDs for shorter context menu labels (GitLens pattern)
• Remove "Diagnostics" submenu — inline commands directly in sidebar context menu

[jeremyruppel]

Very nice!