[kernel-1028] add api clipboard support

[archandatta]

Note

Medium Risk
Introduces clipboard read/write operations (potentially sensitive data) and changes error propagation behavior for invalid --viewport, which could affect scripts relying on prior exit codes/output.

Overview
Adds clipboard support to the kernel browsers computer CLI by wiring the SDK’s new ReadClipboard/WriteClipboard methods through to two new commands: read-clipboard (optionally --output json) and write-clipboard --text ....

Also changes browser creation viewport parsing failures to return a proper error (invalid viewport format: ...) instead of printing to stderr and returning nil, which will affect exit status and error propagation.

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