fix(cli): remove duplicate error output and suppress usage on runtime errors

[GenerQAQ]

Why we need this PR?

Running acontext whoami (or dash ping) when not logged in prints the error message twice — once by Cobra automatically and once by manual fmt.Fprintf in main(). Additionally, runtime errors incorrectly show Usage/help text.

Describe your solution

1. Remove duplicate error print — main.go had fmt.Fprintf(os.Stderr, "Error: %v\n", cmdErr) which duplicated Cobra's built-in error output. Removed it; Cobra handles the printing.
2. Add SilenceUsage: true on root command — runtime errors (e.g. API failures, not-logged-in) should not print Usage info. Usage is only useful for argument/syntax errors, which Cobra handles before RunE.
3. whoami not-logged-in is informational, not an error — runWhoami now prints the message and returns nil instead of returning an error. This matches runLogout's pattern which already does the same.
4. dash ping state checks — "no project selected", "no API key", and ping failure now print info and return nil instead of returning errors. The ping failure case was double-printing (diagnostic output + separate error return).

Implementation Tasks

• Add SilenceUsage: true to rootCmd in main.go
• Remove manual fmt.Fprintf(os.Stderr, ...) error print in main()
• Change runWhoami to treat not-logged-in as info output
• Fix dash ping double-print and state-check error returns

Impact Areas

• CLI Tool

Checklist

• Open your pull request against the dev branch.
• All tests pass in available continuous integration systems (e.g., GitHub Actions).
• Tests are added or modified as needed to cover code changes.

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