[pratikbuilds]

There is now one way to build chat UI: the Conversation family. Remove ChatThread, ChatComposer, ChatInput, ChatPanel, MessageBubble, ApprovalCard, ToolBlock, ToolNarrative, ReasoningBlock and the modules built only on them, with their stories, tests and browser specs. The part types move into the conversation module unchanged, so their names stay stable. ToolProviderMark takes a tool name instead of a part.

README gains "Upgrading from 0.3", which maps every removed export to what replaced it.

Stacked on #15: review only this commit. Part of corbitsdev#94.

Follow-ups (not in this stack)

• The channels-panel badge measures 4.38:1 on stacked 10% washes (recorded as color-contrast-measured in the known list); fix in that component.
• DitherCanvas could import the shared BAYER_4 matrix.
• Share sizing between PromptInputTextarea and the existing Textarea once the old family is gone.
• A parts renderer for the new contract: each host (the whole-chat story, the activity stories, the e2e fixtures) maps ConversationPart to components by hand today.
• Approval fold focus: every host writes the handoff; a family-level helper (e.g. in Activity) would remove that burden.

Verification

• This commit passes bun run typecheck and bun run lint on its own (the stack is checked commit by commit).
• At the top of the stack: bun run check, bun run build, bun run dep-guard, bun run contrast-test and bun run test:e2e (Playwright specs and the Storybook axe sweep in light and dark) pass.

Closes corbitsdev#110

[pratikbuilds]

Moved upstream: corbitsdev#128