feat(ui): govern chat conversation-flow display onto @maka/ui Base UI primitives

[Astro-Han]

Context

The renderer's interactive primitives (packages/ui/src/primitives/*) are already on the shadcn Base UI + Tailwind substrate (cva, data-slot, @base-ui/react). The chat conversation-flow display layer is the last bespoke island: packages/ui/src/components.tsx (MessageBody ~L4441, TurnSummary ~L4618, TurnView ~L4697, ToolActivity ~L6165) renders hand-written maka-* classNames whose CSS is spread across: styles/tool-output.css, styles/tool-stream.css, chat selectors in maka-tokens.css, .maka-turn-* in styles/settings/models.css (model-catalog preview), and .maka-bubble-truncated in styles/reasoning-panel.css. (styles/chat-message.css is the empty/hero state, not the main flow.)

Goal: migrate the conversation flow (bubbles, status/tool markers, tool/result/preview surfaces) onto a small set of @maka/ui chat primitives (Message/Bubble/Marker/Attachment), retiring the bespoke shell CSS, with zero visual change. This is governance (one substrate + a test net), not a redesign; the visual refresh is a separate follow-up.

Invariants (every slice)

• Zero visual change, locked by computed-style/cascade contract tests + before/after screenshots.
• Keep the neutral user-bubble token path (--chat-user-bg / --user-message-bubble); never switch it to primary/accent.
• Replace only the bubble/row shell. Markdown prose under .maka-bubble-assistant (task lists, code blocks, tables, blockquotes, links; maka-tokens.css:1088+) is OUT of scope and must be preserved. Do not touch .messages (chat scroll container).
• No change to runtime, streaming, Markdown rendering, permission, turn model, tool output, attachment IPC.
• New primitives live in packages/ui/src/primitives/chat* and are exported from packages/ui/src/index.ts; do not grow the components.tsx sink.
• Each slice deletes its retired CSS. scripts/check-dead-css.mjs --check only scans styles/ + reference-shell.css (NOT maka-tokens.css); the renderer-style-pruning-contract test does reach tokens via styles.css imports. Lean on contract tests + manual grep of maka-tokens.css.

Slices (flat PRs off main, landed in order)

• PR1 (feat(ui): govern chat bubble/row shell onto @maka/ui chat primitives #334) — Bubble/Message: add chat.tsx Message+Bubble + contract; wire MessageBody; delete the bubble/row shell (.maka-bubble-user, assistant wrapper layout, .message.user/.assistant/.system, .maka-bubble-truncated). Keep Markdown prose.
• PR2 (feat(ui): migrate chat turn markers onto @maka/ui Marker primitive (#332) #337) — Marker: turn summary / failed / aborted / lineage / footer (+ decide maka-turn-thinking: include or explicitly defer) → Marker; delete .maka-turn-* in BOTH maka-tokens.css/tool-output.css AND styles/settings/models.css.
• PR3a (feat(ui): migrate tool live-output stream onto @maka/ui chat primitives (#332 PR3) #348) — Tool live-output stream: ToolOutputStream + the live dot (LiveIndicator) onto chat primitives; deleted .maka-tool-output-stream-* + its per-feature @keyframes. Added canonical @keyframes maka-pulse (shared motion home) + npm run check:chat-visual for the zero-visual diff. LiveIndicator/streamVariants kept internal (off-barrel) until a 2nd consumer.
• PR3b (feat(ui): migrate ToolActivity card shell onto @maka/ui chat primitives (#332 PR3b) #350) — Tool activity: ToolActivity card shell onto internal toolVariants; deleted the .maka-tool* card CSS (+ .toolInline/.toolItem/.toolArgs) and re-keyed the entrance / <summary> marker / pulse-ring residue onto [data-slot="tool"]. The error-banner half was carved out → PR3c. toolVariants kept internal (off-barrel) until a 2nd consumer.
• PR3c (feat(ui): retire ToolErrorBanner bespoke CSS onto the @maka/ui Alert primitive (#332 PR3c) #355) — Tool error banner: retire .maka-tool-error* (ToolErrorBanner already renders on the Alert primitive — this is CSS retirement, not a new primitive). Delete the dead -icon/-body/-title; literalize the live container / -text / -copy tints; sync visible-copy-hygiene-contract.
• PR4 (feat(ui): govern tool-result preview surfaces onto @maka/ui previewVariants (#332 PR4) #362) — Preview surfaces: terminal / diff / web-search / office / explore / subagent overlay previews + the shared .maka-overlay-preview base and the .maka-load-tool-* card. Resolved the shared-.maka-tool-diff* note by co-migrating artifact-preview.tsx onto the barrel-exported previewVariants diff parts (no temporary shared layer). @keyframes maka-tool-card-enter moved to maka-tokens.css.
• Later — MessageScroller: scroll/virtualization (overlay-scroll-area) is a behavioral concern, evaluated separately.

Next component after this batch

• Composer — input-side component. CSS spans styles/composer.css AND a chunk in styles/tool-output.css.

Out of scope (stays hand-written)

Design tokens (maka-tokens.css), Markdown prose, feature-page layout CSS (sidebar / settings / module-pages / onboarding / etc.), and no-equivalent primitives (overlay-scroll-area, frame, kbd, skeleton, spinner, empty, alert, input-group).