Add deferred search content command

[chruffins]

Summary

• add kernel search contents <id> with a JSON request body for selected result content
• validate the required result selector before sending the request
• preserve the API response as formatted JSON

Testing

• go test ./cmd -run TestSearch

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Note

Medium Risk
New CLI surface can trigger billable search content and browser retrieval; validation is limited to the result selector, with remaining fields passed through to the API.

Overview
Adds kernel search contents <id>, a subcommand that POSTs to search/<id>/contents with a required --request JSON body so users can pull page or provider content for a retained search after the initial query.

Client-side validation rejects missing or malformed --request payloads and enforces exactly one of result_ids or limit before the API call. Help text notes that browser-backed retrieval can use concurrency and incur charges. Responses still flow through the existing executeSearchRequest path (indented JSON on stdout, no retries on POST).

Tests cover a happy-path POST/body assertion, extra invalid-input cases for the new command, and command wiring alongside get and providers.

^{Reviewed by Cursor Bugbot for commit 8a61f40. Configure here.}

[chruffins]

Closing as superseded by #264, which merged the search contents command and its typed flags.