fix(session-ui): restore compaction summaries

[Hona]

Summary

• restore the production Session compacted divider for current compaction messages instead of rendering them as generic notices
• stream the compaction summary through the normal assistant Markdown presentation
• preserve production failure behavior with normalized error cards and quiet cancellation
• add Storybook coverage for running, completed, failed, and cancelled compactions

Testing

• pre-push full workspace typecheck
• session-ui typecheck and 95 tests
• app and E2E typechecks
• focused compaction E2E regression tests: 2 passed
• full session timeline Notice regression file: 10 passed
• production Storybook build

Performance

• attempted the focused production streaming-timeline benchmark before and after the change
• both runs failed before the scenario because the fixture session title never appeared, so the harness produced no comparable metrics

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[Enough1122]

AI code review — automated review for reference, author can ignore or act on any point.

Nice, focused restoration. The two regression tests exercise the full event lifecycle (started → delta → ended / failed / aborted) through the real reducer, including replacement of streamed text by the final summary and hiding of cancellation detail — good coverage. Removing the compaction branch from the notice-describe function is also the right call: otherwise these rows would render both the old one-line notice and the new full component.

A few points to consider:

1. packages/session-ui/src/message/message-content.tsx (SessionCompactionMessage, content()): for a failed, non-aborted compaction you render props.message.error.message verbatim through PacedMarkdown. Provider error strings are arbitrary text; sequences like _underscores_, backticks, or HTML-ish tags will render oddly (or be interpreted) instead of shown literally. Consider plain-text styling for failure details and reserving markdown rendering for real summaries.

2. Same file: with streaming={props.message.status === "running"}, a long-running compaction that never emits a delta shows only the divider — fine — but if the stream stalls mid-delta there's no visual distinction between "still streaming" and "hung". Not blocking; just something the failed/cancelled states don't cover.

3. packages/ui/src/i18n/en.ts: the three new keys are English-only. ~56 sibling locales fall back to en today, so users of those locales will see mixed-language dividers ("Compaction failed") until translations land. If there's a translation-sync process, worth flagging the keys there.

4. packages/app/e2e/regression/session-timeline-notices.spec.ts: the subagent test swapped toolPart("call_subagent", "subagent", "streaming", {}) for an inline literal with input: "". If that was forced by a type change, consider updating the shared toolPart fixture signature instead — otherwise the next test needing a streaming tool part hits the same mismatch and copies the inline workaround.

5. Minor: the aria-label assertion reformat in the same test file looks like Prettier fall-out; unrelated to the fix but harmless.