[pratikbuilds]

Add src/lib/conversation.ts, the message model and part union every chat component reads: a tool part carrying an approval record beside the call, parallel tool batches, generated images, sources, a streaming status, replies and reactions. It reuses the chat-parts.ts members unchanged.

Add groupActivity and summarizeActivity to fold thinking and tool steps into one run, mediaKindOf, and checkAttachments for count, size and duplicate limits. Add the --attention-wash token, mixed from --primary.

First in the stack; based on main at 8300019. Part of corbitsdev#94.

Notes for review

• --attention-wash mixes 12% of --primary (reference design: 11% light / 14% dark; one value serves both).
• Summary durations are whole seconds ("4s", "1m 05s"); the old parts-renderer shows "4.0s" until it is removed.
• An approval resolving merges the activity runs on either side; the later run's key changes (documented, tested).

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#95

[pratikbuilds]

Moved upstream: corbitsdev#113