Chat: legible tool activity in the conversation

[TheGreatAxios]

What a turn with tool calls looked like

Before. In the transcript, each tool-trace part rendered as its own
react-ui ToolBlock, titled with the tool's identifier run through a
humaniser — mcp_read became "Mcp read", slack__post_message became
"Post message (Slack)", so every MCP call in a conversation read alike.
Opening one showed JSON.stringify(input, null, 2) and the result
stringified the same way. A failed call opened itself by default, so a
raw JSON error blob sat expanded inline in the conversation. Five
consecutive calls stacked as five blocks between the question and the
answer.

In the live strip, a call showed the bare tool name plus elapsed seconds,
and tool.done marked it "done" regardless of isError — a tool that
failed settled into the same quiet marker as one that succeeded. A failure
was invisible until the turn ended.

After. One vocabulary (tool-activity.ts) serves both surfaces:

• Naming — a call renders as what it accomplished: Searched the web for "pricing", Wrote a file — report.md, Posted a message in Slack #general, Saved an issue in Linear. The generic MCP dispatch tools
  resolve to the tool they actually invoked. No identifier, no namespace,
  no underscore reaches the DOM.
• No JSON, ever — arguments are folded into the sentence rather than
  dumped. A result becomes plain text: the prose the tool returned, or a
  count (2 results., Nothing found.) when it returned a list. An
  opaque object yields no detail at all rather than a stringified blob.
• In-progress — tense follows state. Searching the web for "x" while
  it runs, Searched the web for "x" once it settles. A running row
  carries elapsed seconds; a settled one drops the timer.
• Failure — settles as failed, is the only state here that earns
  colour, and opens onto the reason in words. A tool that failed without
  saying why still says No reason given.
• Grouping — consecutive calls fold into one round on a single line:
  the step currently working while the turn is open, then 3 steps once
  settled, or 3 steps, 2 didn't work when some failed. Collapsed by
  default so the reply stays above its own machinery; a round containing a
  failure opens itself.

The live strip and the persisted transcript now render through the same
rows, so a call does not restyle itself the moment its turn ends.

Why ToolBlock is no longer used here

react-ui's ToolBlock renders one call at a time and stringifies both its
arguments and its output — the two things this change exists to stop. The
row idiom it is replaced with is workbench composition (grouping into
rounds, the consumer-language vocabulary) and matches the chat surface's
own radius-0 / Phosphor / quiet-chrome canon. ToolBlock stays untouched
upstream; toReactUiToolTrace is removed here rather than left beside the
new path. toReactUiReasoning and react-ui's reasoning disclosure are
unchanged.

Test plan

• bun run typecheck in packages/chat-ui — clean
• bun test in packages/chat-ui — 648 pass, 0 fail
• eslint / prettier clean on touched files
• New src/tool-activity.test.ts (28 cases): identity resolution, phrasing
  in both tenses, output-to-plain-text extraction, failure summaries, round
  summaries, part grouping.
• New test/tool-activity-view.test.tsx (8 cases, mounted for real):
  success reads as a sentence with no identifier and no { in the DOM;
  detail stays closed until clicked; a failure marks itself and opens onto
  its reason; a failure with nothing to say still says so; a running call
  is present-tense and offers no disclosure; three consecutive calls
  collapse to 3 steps and open onto all three; a still-working round
  shows the working step; a round with a failure names it.

Not verified

• Not exercised against a live agent turn on a running stack (owner is
  testing live on :3000; this lane did not touch that stack or its DB).
  Everything above is component/state level.
• Visual review is by code and DOM assertions only — no screenshot pass.
• packages/folded-runs was audited (inferenceDoneBlocks,
  toolDoneResult) and found correct for this work; it is unchanged.
• PR CL-6432: turn-phase typing indicator (reopened) #183's turn-phase state machine in streaming-reply.ts is untouched.