feat(subagent): open a child's chat in the native right sidebar

The host's own catalog puts an "open beside" action on every non-current
child row; this port had never wired it. Add it: the row grows a hover
button that opens the child's chat as a `subagentchat` resource in the
native right Sidebar, so a user can watch a child work without giving up
the pane he is in (the in-pane jump stays the plain row click).

- src/client/native/subagent-chat.ts: the resource-address codec, copied
  from @deepseek-ai/dsh-client-ui-subagent rather than imported (the
  package exports no helpers and the plugin must not reach into it).
- native/surface.ts: carry `kind` / `preferNewPane` on an open.
- SubagentView: `openChildAside` via `ctx.get?.('sidebarRight')` — an
  optional probe, since a fake ctx without `get` otherwise crashes the
  whole render (9 specs caught it) — threaded through all three
  CatalogRows call sites.
- locales: `subagentOpenAside` in zh/en plus all 20 third-language
  dictionaries (tests/locales.spec.ts enforces key-set parity).
- docs: the conventions page now documents the resource-open recipe.

Gates: typecheck / lint / build / test 132 files 1390 passed /
check:consumer-types, plus the repo's runtime-contract symbol check.

tests/subagent-chat-aside.spec.ts

@@ -0,0 +1,52 @@
+/**
+ * The `subagentchat` resource address must stay byte-compatible with the host's
+ * own codec: `@deepseek-ai/dsh-client-ui-subagent` parses what we emit, so a
+ * different query order or an unescaped child id would silently open the wrong
+ * page (or none).
+ */
+import { describe, expect, it } from 'vitest'
+import {
+  SUBAGENT_CHAT_ADDRESS,
+  SUBAGENT_CHAT_KIND,
+  subagentChatAddress,
+} from '../src/client/native/subagent-chat.ts'
+
+describe('subagentChatAddress', () => {
+  it("emits the canonical form with parent and mode", () => {
+    expect(subagentChatAddress({
+      parentSessionId: 'parent-1',
+      childSessionId: 'child-2',
+      mode: 'one-shot',
+    })).toBe('dsh-resource://subagentchat/session/child-2?parent=parent-1&mode=one-shot')
+  })
+
+  it('keeps a continuable mode verbatim', () => {
+    expect(subagentChatAddress({
+      parentSessionId: 'p',
+      childSessionId: 'c',
+      mode: 'continuable',
+    })).toContain('mode=continuable')
+  })
+
+  it('percent-encodes a child id so the address stays one path segment', () => {
+    const address = subagentChatAddress({
+      parentSessionId: 'p',
+      childSessionId: 'child/with:odd chars',
+      mode: 'one-shot',
+    })
+    expect(address).toContain('/child%2Fwith%3Aodd%20chars?')
+  })
+
+  it('never leaks the parent into the path (it is a routing query only)', () => {
+    const address = subagentChatAddress({
+      parentSessionId: 'p secret',
+      childSessionId: 'c',
+      mode: 'one-shot',
+    })
+    expect(address.split('?')[0]).toBe(`${SUBAGENT_CHAT_ADDRESS}c`)
+  })
+
+  it('names the page kind the native Sidebar roots these pages under', () => {
+    expect(SUBAGENT_CHAT_KIND).toBe('subagentchat')
+  })
+})