feat(tabs): confirm dialog that cascades a tab close to its subagents

Closing a tab whose visible subagent group is non-empty now asks ONE
daisyUI modal (CloseSubagentsConfirm): 'Yes, close all N' closes the tab
plus every transitive descendant tab (sub-sub-agents included, no further
prompts), 'No, keep them open' closes only the tab, and Cancel/Escape/
backdrop closes nothing. Plain tabs close directly as before.

- cascadeCloseIds (pure, tab-groups.ts): the tab + its transitive visible
  group descendants in tab order — one decision covers the whole subtree
- TabList intercepts tab-close and opens the dialog only when the
  subtree has >1 tab; ids are snapshotted at open time
- each closed conversation goes through the existing onClose port, so
  every close still POSTs /conversations/:id/close (backend doesn't
  cascade; it is idempotent)

1 files changed, 20 insertions, 0 deletions

diff --git a/notes/assumptions-log.md b/notes/assumptions-log.md
index 728f8db..c6daf59 100644
--- a/notes/assumptions-log.md
+++ b/notes/assumptions-log.md
@@ -2,6 +2,26 @@
 
 > Recorded while working autonomously (user away). Raise these when the user returns.
 
+## 2026-10-16 — Close-tab cascade confirm dialog (subagent groups)
+
+1. **"Subagents" of a tab = its VISIBLE subagent group (open tabs only).** The confirm dialog
+   triggers when the closed tab has descendants in the tab grid's grouping
+   (`tabGroupParentId` forest, transitive). Subagent conversations that were never opened as
+   tabs are NOT closed — the FE has no knowledge of them (no lineage API consumed here). The
+   backend's `POST /conversations/:id/close` does not cascade either, so this matches the
+   visible mental model.
+
+2. **The cascade issues one `POST /close` per conversation** (via the existing `onClose` →
+   `store.closeTab` port, unchanged). The backend close is idempotent, so a descendant closed
+   by another client mid-dialog is a harmless no-op. No new cross-unit contract was needed:
+   the decision logic is pure (`cascadeCloseIds` in the tabs unit) and the dialog lives inside
+   `TabList`.
+
+3. **Dialog offers three outs:** "Yes, close all N" (cascade), "No, keep them open" (close
+   only the target tab — the pre-change behavior), and Cancel/Escape/backdrop (close nothing).
+   The user asked for a yes/no prompt; Cancel was added as the safe dismissal path. The
+   cascade-order snapshot (ids) is taken when the dialog opens.
+
 ## 2026-06-24 — CR-10: workspaceId on conversation.open / statusChanged
 
 1. **Contract version already at 0.19.0 in the backend repo.** The `file:` dep in `package.json`