Add --json, --jq, and --template flags to `gh agent-task view`,
consistent with the pattern used by `gh pr view --json`,
`gh issue view --json`, etc.
This reuses the same ExportData interface and SessionFields defined
for list, applying them to the single-session view output.
Closes https://github.com/cli/cli/issues/12805 (partial)
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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