prompt-input: adopt CSS field-sizing:content auto-grow in the primitive

[rickylabs]

Problem. The composer needs an auto-growing textarea. Modern baseline CSS field-sizing: content replaces JS autosize entirely (now cross-browser). eis-chat applies it via app CSS (session.css) on top of PromptInput.

Proposal. Move field-sizing: content into the prompt-input primitive CSS so every consumer gets zero-JS auto-grow by default.

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Surfaced during the eis-chat v2 refactor (netscript alpha.18). Promotion candidate → @netscript/fresh-ui / web-layer.