Chat tool chips: conform to the spec

[TheGreatAxios]

Summary

PR #190 shipped tool-use chips that diverge from the authoritative mock
spec (mock-spec.md §12.3, §1) on nearly every visual dimension, and edited
DESIGN.md to document its own divergence as canon. This brings the chip
back to spec. Ref CL-6466.

Findings fixed, in order from the review:

1. Chips are not collapsibles. Removed describeToolRound and the
   round-folding UI. ToolActivityGroup now stacks every tool call as its
   own chip, per §12.3 — no group-level trigger, no folding ≥2 calls into
   one collapsed line.
2. "3 steps" removed. That count made three benign file reads and a
   round containing slack__post_message render identically — a reader
   couldn't tell something public happened without clicking, and nothing
   invited the click. Gone along with the folding.
3. Provider tile added. 22×22, brand-colored, two-letter, per §12.3's
   anatomy (provider tile · tool name · argument summary + elapsed · status pill). providerTile() in tool-activity.ts maps known
   providers (Slack, GitHub, Linear, …) to their brand color/initials, with
   a neutral fallback for anything else.
4. Entrance animation added. Chips animate in with the same rowIn-
   style keyframe list rows use elsewhere in chat-ui, gated behind
   prefers-reduced-motion: no-preference.
5. Hit area fixed. .chat-tool-activity-trigger extends to a 40px
   tall target via a ::before pseudo-element without growing the visible
   chip.
6. Token violations fixed. Chip/detail radius now uses var(--radius)
   instead of a hardcoded 0 (§1: radius is 8/10/12/99, nothing else).
   The caret icon is now sized by font-size on the svg rather than
   width/height (§1's icon-sizing rule).
7. DESIGN.md's self-canonization reverted. The "Tool Activity in the
   Conversation" section now describes the spec's actual chip contract
   instead of documenting the folding behavior as canon.

Left alone, as directed: the endsWith("s") pluralization heuristic in
objectPhrase, and the prose-sentence layer in tool-activity.ts (tense,
naming, failure summaries, no-JSON guarantee) — that layer is good and is
unchanged; only presentation/grouping changed.

Note for the reviewer

apps/web/src/app.css sets --radius: 0 app-wide (a pre-existing,
separately-documented decision predating this PR). The chip CSS now uses
var(--radius) per §1's token rule, which is the spec-correct fix at the
component level, but that global override means the chip will still
render square-cornered in the live app until/unless that broader app-wide
override is revisited — out of scope for this corrective pass.

Test plan

• bun run typecheck — passes (had to add the new provider field to
  the one other manual ToolActivityRow construction, in
  turn-activity.tsx)
• bunx eslint . — no new errors (chat-ui scope)
• bunx prettier --check . — passes (chat-ui scope)
• bun test — 650 pass, 0 fail (chat-ui scope)
• Updated the tests that asserted the removed round-folding instead of
  deleting coverage; added tests for providerTile

DO NOT MERGE — reporting back for peer review per the corrective-lane
process.