fix(subagent): stop 401-ing the aside-open with a guessed child mode

The aside button could hit `subagent/unauthorized: subagent mode does not
match the supplied address`. The host's session-controller (dsh-api-session-
controller validateAddress) compares the `mode` in the resource address
against the child's durable descriptor mode and rejects any mismatch. The
address `mode` came from the row, and on the degraded path the row's mode is
a best-effort guess from the summary `running` flag (running => continuable)
— so a child that is genuinely one-shot but still running was guessed as
continuable and the very open it was meant to trigger 401'd.

Two layers, so it holds on both the real-catalog and the degraded path:

- subagentChatAddress: only pass a mode through when it is one of the two
  durable values (`one-shot` / `continuable`). Anything else is omitted,
  which the controller reads as `mode: "unknown"` and therefore skips the
  comparison — the safest form when the caller does not actually know.
- openChildAside: prefer the host's real descriptor mode for the child
  (the `subagentsByParent` entry carries the durable mode) over the address
  mode, falling back to the address only when the real catalog never arrived.

Gates: typecheck / lint / build / test 132 files 1392 passed.

src/client/SubagentView.tsx

@@ -855,11 +855,20 @@ export function SubagentView(props: {
    * `openChild`; this one never switches the main pane.
    */
   const openChildAside = useCallback((address: SidebarSubagentAddress): void => {
-    column?.openResource?.(subagentChatAddress(address), {
+    // The address's `mode` may be an inference (the degraded synthesized
+    // catalog). Prefer the host's REAL descriptor mode for this child — the
+    // `subagentsByParent` entry carries the durable `mode` the session-
+    // controller validates against — and only fall back to the address when
+    // the real catalog never arrived.
+    const entry = catalogs[address.parentSessionId]?.entries.find(
+      (e): e is SidebarSubagentChildEntry => e.kind === 'child' && e.id === address.childSessionId,
+    )
+    const mode = entry?.mode ?? address.mode
+    column?.openResource?.(subagentChatAddress({ ...address, mode }), {
       kind: SUBAGENT_CHAT_KIND,
       preferNewPane: true,
     })
-  }, [column])
+  }, [column, catalogs])
 
   const totals = useMemo(
     () => rootId === undefined

src/client/native/subagent-chat.ts

@@ -11,6 +11,9 @@
  */
 import type { SidebarSubagentAddress } from '../../context-types.ts'
 
+/** The only modes the host's session-controller will accept in an address. */
+const KNOWN_MODES = new Set(['one-shot', 'continuable'])
+
 /** Resource-address prefix for an embedded Session chat. */
 export const SUBAGENT_CHAT_ADDRESS = 'dsh-resource://subagentchat/session/'
 
@@ -24,9 +27,16 @@ export const SUBAGENT_CHAT_KIND = 'subagentchat'
  * @returns canonical Sidebar resource address.
  */
 export function subagentChatAddress(address: SidebarSubagentAddress): string {
-  const query = new URLSearchParams({
-    parent: address.parentSessionId,
-    mode: address.mode,
-  })
+  // `mode` is a GATE, not a label: the host's session-controller rejects an
+  // address whose `mode` differs from the child's durable descriptor mode
+  // (`subagent/unauthorized: subagent mode does not match the supplied
+  // address`). The row-level `mode` can be a best-effort inference (the
+  // degraded summary-synthesized catalog guesses from `running`), so a wrong
+  // guess used to 401 the very open it was meant to trigger. A valid mode is
+  // passed through verbatim; anything else is OMITTED, which the controller
+  // reads as `mode: "unknown"` and therefore skips the comparison entirely —
+  // the safest form when the caller does not actually know the mode.
+  const query = new URLSearchParams({ parent: address.parentSessionId })
+  if (KNOWN_MODES.has(address.mode)) query.set('mode', address.mode)
   return `${SUBAGENT_CHAT_ADDRESS}${encodeURIComponent(address.childSessionId)}?${query.toString()}`
 }

tests/subagent-chat-aside.spec.ts

@@ -49,4 +49,25 @@ describe('subagentChatAddress', () => {
   it('names the page kind the native Sidebar roots these pages under', () => {
     expect(SUBAGENT_CHAT_KIND).toBe('subagentchat')
   })
+
+  it('omits an unknown mode so the host treats it as "unknown" and skips the check', () => {
+    // The degraded synthesized catalog guesses mode from `running`; a wrong
+    // guess used to 401 the open (the controller compares address.mode against
+    // the child's durable descriptor). An unrecognized mode must be dropped,
+    // which the controller reads as `mode: "unknown"` and does not compare.
+    const address = subagentChatAddress({
+      parentSessionId: 'p',
+      childSessionId: 'c',
+      mode: 'unknown' as never,
+    })
+    expect(address).toBe('dsh-resource://subagentchat/session/c?parent=p')
+    expect(address).not.toContain('mode=')
+  })
+
+  it('still emits the mode when it is one of the two durable values', () => {
+    expect(subagentChatAddress({ parentSessionId: 'p', childSessionId: 'c', mode: 'one-shot' }))
+      .toContain('mode=one-shot')
+    expect(subagentChatAddress({ parentSessionId: 'p', childSessionId: 'c', mode: 'continuable' }))
+      .toContain('mode=continuable')
+  })
 })