refactor(client): 统一 GitLens 错误归一化与 SubagentView 日志分级

- GitLens.tsx：六处 catch 里的 reason instanceof Error ?
  reason.message : String(reason)（refreshTarget/refresh/loadMoreLog/
  commit/checkout/runConfirmed）收敛为模块级 errorMessage()，
  两处模板字符串插值场景（historyLoadError/checkoutError 前缀）
  同样复用；归一化表达式与输出文本逐字不变。
- SubagentView.tsx：openSubagent/open session 失败的 console.warn
  改为 console.error，与同严重度失败路径对齐（Sidebar.tsx tab
  badge、TerminalView.tsx xterm open、service.ts urlTarget/plugin
  callback 均为 console.error）；前缀与消息文本不变。tests/ 无任何
  钉 console.warn 的断言，已核实。

src/client/SubagentView.tsx

@@ -715,7 +715,7 @@ export function SubagentView(props: {
     try {
       sessions.openSubagent?.(address)
     } catch (error) {
-      console.warn('[dsh-better-sidebar] openSubagent failed:', error)
+      console.error('[dsh-better-sidebar] openSubagent failed:', error)
     }
   }, [sessions, onOpenChild])
 
@@ -725,7 +725,7 @@ export function SubagentView(props: {
     try {
       sessions.open?.(rootId)
     } catch (error) {
-      console.warn('[dsh-better-sidebar] open session failed:', error)
+      console.error('[dsh-better-sidebar] open session failed:', error)
     }
   }, [sessions, rootId])
 

src/client/changes/GitLens.tsx

@@ -64,6 +64,12 @@ function refNames(refs: string): string[] {
   )]
 }
 
+/** One thrown value as display text (every error banner/row here normalizes
+ *  through this so non-Error rejections never render as '[object Object]'). */
+function errorMessage(reason: unknown): string {
+  return reason instanceof Error ? reason.message : String(reason)
+}
+
 /** The pending destructive action (discard / revert / cherry-pick), gated by a confirm modal. */
 interface ConfirmState {
   title: string
@@ -157,7 +163,7 @@ export function GitLens(props: GitLensProps) {
       setLogEnded(logResult.length < LOG_BATCH)
     } catch (reason) {
       if (options.generation === refreshGeneration.current) {
-        setError(reason instanceof Error ? reason.message : String(reason))
+        setError(errorMessage(reason))
       }
     } finally {
       if (options.loading && options.generation === refreshGeneration.current) setLoading(false)
@@ -219,7 +225,7 @@ export function GitLens(props: GitLensProps) {
       await refreshTarget(target, { loading: !silent, generation })
     } catch (reason) {
       if (generation === refreshGeneration.current) {
-        setError(reason instanceof Error ? reason.message : String(reason))
+        setError(errorMessage(reason))
         if (!silent) setLoading(false)
       }
     } finally {
@@ -289,7 +295,7 @@ export function GitLens(props: GitLensProps) {
       if (next.length < LOG_BATCH) setLogEnded(true)
     } catch (reason) {
       if (generation === refreshGeneration.current && target === chosenPathRef.current) {
-        setCommitError(`${t('historyLoadError')}: ${reason instanceof Error ? reason.message : String(reason)}`)
+        setCommitError(`${t('historyLoadError')}: ${errorMessage(reason)}`)
       }
     } finally {
       if (generation === refreshGeneration.current && target === chosenPathRef.current) setLogLoadingMore(false)
@@ -355,7 +361,7 @@ export function GitLens(props: GitLensProps) {
       setCommitMsg('')
       await refresh()
     } catch (reason) {
-      setCommitError(reason instanceof Error ? reason.message : String(reason))
+      setCommitError(errorMessage(reason))
     } finally {
       setBusy(false)
     }
@@ -369,7 +375,7 @@ export function GitLens(props: GitLensProps) {
       await api.gitCheckout(gitScope, branch, selectedWorktree)
       await refresh()
     } catch (reason) {
-      setCommitError(`${t('checkoutError')}: ${reason instanceof Error ? reason.message : String(reason)}`)
+      setCommitError(`${t('checkoutError')}: ${errorMessage(reason)}`)
     } finally {
       setBusy(false)
     }
@@ -384,7 +390,7 @@ export function GitLens(props: GitLensProps) {
         await confirmState.onConfirm()
         await refresh()
       } catch (reason) {
-        setCommitError(reason instanceof Error ? reason.message : String(reason))
+        setCommitError(errorMessage(reason))
       } finally {
         setBusy(false)
       }