terminal: show recv wait/max params + bash [bg] flag

recv tool-use rows rendered as a bare recv() regardless of args,
hiding whether a turn is parked on a long-poll (wait_seconds) or
draining a burst (max). fmtToolUse now surfaces both. Bash rows
gain a [bg] flag when run_in_background is set.

closes #158

docs/terminal-rendering.md

@@ -69,6 +69,10 @@ parent's negative pull.
      `renderRichToolUse` (Write/Edit/send/ask/answer get
      custom renderings); on miss fall through to a flat
      `.tool-use` row with `fmtToolUse → fmtArgsGeneric`.
+     `fmtToolUse` surfaces the salient arg per built-in tool —
+     e.g. `recv` shows `wait <N>s` / `max <N>` when set (bare
+     `recv()` otherwise), `Bash` flags `[bg]` for
+     `run_in_background` commands.
 4. `type == "user"` → walk `message.content[]` for
    `tool_result`; `renderToolResult` correlates via
    `tool_use_id → toolNameById` to default-open `recv`