Simplify terminal message shape to a uniform TermMsg

Move terminal-row classification server-side into a new
hive-agent/src/term_msg.rs, replacing the old JSON-mutation
enrich()/stamped-field approach in stream_enrich.rs with one
uniform wire shape: {icon?, level: debug|info|warn|error, summary,
body?, body_format?: markdown|diff, coalesce_key?}. No more per-row
`kind` tag or raw claude-JSON passthrough — every row is the same
shape, with structural identity carried by icon + summary text
instead of a CSS class per row kind.

hive-agent/src/web_ui/stream.rs's history + SSE endpoints now both
call term_msg::classify() and serve TermEnvelope{ts, seq?, msgs}
frames; events that classify to zero rows (agent-state changes,
drop-noise) never reach the wire.

Frontend: classifyEvent.ts collapses from a large per-tool dispatch
tree to a thin TermMsg -> StreamRow adapter. streamRow.ts/Row.tsx
drop the now-dead meta/childText fields. terminal.css switches from
a dozen-odd per-row-kind classes to four level-based color rules.
Expand/collapse of a bodied row is now a uniform client-side
decision (the operator's preference), no server-side per-tool
override.

docs/terminal-rendering.md rewritten to match.

frontend/packages/agent/src/components/Row.tsx

@@ -59,12 +59,6 @@ export function Row({ row }: { row: StreamRow }) {
     <div className={`row ${row.cssClass}`}>
       {row.icon != null && row.icon !== '' && <span className="row-glyph">{row.icon}</span>}
       {row.text != null && linkifyToNodes(row.text)}
-      {row.meta?.map((m) => (
-        <span key={m.cls} className={m.cls}>
-          {m.text}
-        </span>
-      ))}
-      {row.childText != null && <div className={row.childText.cls}>{row.childText.text}</div>}
       {row.markdownBody != null && <MarkdownBody text={row.markdownBody} />}
     </div>
   );