[kitlangton]

What it does. Removes four session-ui components that nothing renders anymore: MessageNav, ToolStatusTitle, ToolCountSummary and ToolCountLabel, plus the message-part barrel that re-exported other modules. Each one still shipped CSS in the global session-ui stylesheet and showed up in the package exports.

How it works.

1. Delete the components, their CSS, and the ToolCountSummary story.
2. Drop their entries from package.json exports and styles/index.css.
3. Drop the one import-boundaries.test.ts assertion that only read message-part.tsx.

No package imports any of these subpaths on dev or beta.

Risk. Low, because none of it is rendered. Two leftovers I left alone: the i18n key ui.messageNav.newMessage in packages/ui, and the timeline-stability perf spec, which already selected [data-component="tool-status-title"] and matched nothing.

The next dev→beta merge will need a few one-line conflict resolutions, all "keep the deletion":

• beta has small token edits in these four CSS files (modify/delete);
• beta adds a nearby genui line in package.json exports and in styles/index.css.

Proof. No media, since this only deletes code.

• bun typecheck in packages/session-ui: clean
• bun test src/import-boundaries.test.ts: 4 pass
• bun run lint:changed, prettier: clean
• pre-push bun turbo typecheck: 34/34 packages pass