fix(tui): show a visible marker where /rebuild inserted a context boundary

[wqymi]

The UX gap

After a manual /rebuild, the TUI shows nothing. The user cannot tell that a rebuild happened, and cannot tell where the context boundary is.

checkpoint.insertRebuildBoundary (src/session/checkpoint.ts:1444-1490) inserts exactly one role: "user" message whose parts are:

• one { type: "checkpoint", coveredUpTo, ... } part
• two-to-three { type: "text", synthetic: true } parts (the rendered context + index + actor registry)

Neither renders in the TUI:

• PART_MAPPING in src/cli/cmd/tui/routes/session/index.tsx:1845-1849 only maps text, tool, reasoning — a checkpoint part falls through <Show when={component()}> and renders nothing.
• UserMessage only draws its bubble when a non-synthetic text part exists (index.tsx:1513, gated at index.tsx:1615), so all the synthetic context text is hidden too.

Net effect: the whole boundary message renders as zero rows.

Compaction is locatable by comparison, because SessionCompaction.processCompaction writes a real assistant message (agent: "compaction", summary: true, src/session/compaction.ts:341-346) whose text part renders normally — you see the summary block sitting in the transcript. Rebuild has no equivalent: its summary lives inside synthetic: true user text parts, which are never displayed.

(filterCompacted, src/session/message-v2.ts:1029, breaks the context slice on either part type, so both are equally real boundaries — only one of them was visible.)

The fix

Render the checkpoint part as a one-line marker row in UserMessage, reusing the badge-row pattern the TUI already uses for cron fires and actor notifications (same box marginTop/paddingLeft={2}, same backgroundElement badge + muted trailing text):

 ⟲ context rebuilt  earlier messages summarized

Placed inline at the boundary message's position, so it marks where the rebuild happened, not just that it happened.

New i18n keys (en / zh / zht; other locales fall back to English via the base merge in context/language.tsx:18):

• tui.session.rebuild_boundary.label — context rebuilt / 上下文已重建 / 上下文已重建
• tui.session.rebuild_boundary.detail — earlier messages summarized / 较早消息已摘要 / 較早訊息已摘要

No rebuild or compaction logic is touched — this is purely making an already-existing boundary visible.

Note on consistency

The web/share renderer (packages/ui) does have a dedicated divider: PART_MAPPING["compaction"] → MessageDivider label={i18n.t("ui.messagePart.compaction")} (packages/ui/src/components/message-part.tsx:1325-1328), and session-turn.tsx:233 already treats a checkpoint part as a divider trigger. The TUI has no compaction divider at all — compaction is only visible there via its summary message. This PR does not add a TUI compaction divider (out of scope); the new marker follows the TUI's own inline badge-row language rather than importing the web divider.

Verification

• bun typecheck (packages/opencode) — exit 0
• bun test test/cli/cmd/tui/ — 68 pass / 0 fail
• bun test test/session/checkpoint-rebuild-unify.test.ts — 3 pass / 0 fail (this already pins the invariant the marker keys off: expect(boundary.parts.some((p) => p.type === "checkpoint")).toBe(true) at line 168)
• New test/cli/cmd/tui/rebuild-boundary-marker.test.ts pins the marker copy in en/zh/zht and asserts zh/zht are actually translated rather than silently falling back to English. There is no render/snapshot harness for TUI message parts (the existing TUI tests all cover pure helpers), so there was no compaction-marker snapshot test to mirror.

To see it live: open a TUI session with some history, run /rebuild, and the marker row appears inline at the point the boundary was inserted.

[wqymi]

Folding this into #1752 — it is the UX completion of the same /rebuild feature, not a standalone change. Commit 054775d now carries the badge row on #1752's branch, plus a test pinning the model-context semantics behind the decision to keep this render-only.