feat(cli): add whoami / doctor / kb / context commands (PR-7)

5 new leaf commands wired into the root tree:
  whoami      — simplified `auth status` (user_id + tenant_id only)
  doctor      — 4-item self-check (base_url / auth / server_version / cred_storage)
                with --offline / --no-cache flags + skip cascade + summary.all_passed
                防 agent 看到 envelope.ok=true 误判命令整体 success
  kb list     — list KBs (default updated_at desc; 0 KB → "(no knowledge bases)")
                tabwriter 4-col (ID/NAME/DOCS/UPDATED), display-width truncation
  kb get      — show single KB details (KEY: VALUE, suppress empty fields)
  context use — switch default context (writes config.current_context),
                带 levenshtein distance ≤ 2 的 did-you-mean hint

Each command uses the v0.0 narrow Service interface pattern (testable via
fakes), agent.SetAgentHelp for AI-friendly hints, and ClassifyHTTPError
for stable error code mapping.

cmdutil/errors.go: 新增 CodeLocalContextNotFound for `context use`,加入 AllCodes() 注册集.

cli/cmd/root.go: NewRootCmd 改为 exported (acceptance/contract 测试需要),
                 注册 4 个新命令 + 1 parent group; root_test.go 跟随更新.

cli/cmd/kb/list.go

@@ -0,0 +1,82 @@
+package kb
+
+import (
+	"context"
+	"fmt"
+	"text/tabwriter"
+	"time"
+
+	"github.com/spf13/cobra"
+
+	"github.com/Tencent/WeKnora/cli/internal/agent"
+	"github.com/Tencent/WeKnora/cli/internal/cmdutil"
+	"github.com/Tencent/WeKnora/cli/internal/format"
+	"github.com/Tencent/WeKnora/cli/internal/iostreams"
+	"github.com/Tencent/WeKnora/cli/internal/text"
+	sdk "github.com/Tencent/WeKnora/client"
+)
+
+// ListOptions captures `weknora kb list` flags.
+type ListOptions struct {
+	JSONOut bool
+}
+
+// ListService is the narrow SDK surface this command depends on.
+type ListService interface {
+	ListKnowledgeBases(ctx context.Context) ([]sdk.KnowledgeBase, error)
+}
+
+// listResult is the typed payload emitted under data.items.
+type listResult struct {
+	Items []sdk.KnowledgeBase `json:"items"`
+}
+
+// NewCmdList builds `weknora kb list`.
+func NewCmdList(f *cmdutil.Factory) *cobra.Command {
+	opts := &ListOptions{}
+	cmd := &cobra.Command{
+		Use:   "list",
+		Short: "List knowledge bases visible to the active context",
+		Args:  cobra.NoArgs,
+		RunE: func(c *cobra.Command, _ []string) error {
+			cli, err := f.Client()
+			if err != nil {
+				return err
+			}
+			return runList(c.Context(), opts, cli)
+		},
+	}
+	cmd.Flags().BoolVar(&opts.JSONOut, "json", false, "Output JSON envelope")
+	agent.SetAgentHelp(cmd, "Lists all knowledge bases. Returns data.items: [{id, name, ...}]; empty array when none.")
+	return cmd
+}
+
+func runList(ctx context.Context, opts *ListOptions, svc ListService) error {
+	items, err := svc.ListKnowledgeBases(ctx)
+	if err != nil {
+		return cmdutil.Wrapf(cmdutil.ClassifyHTTPError(err), err, "list knowledge bases")
+	}
+	if items == nil {
+		items = []sdk.KnowledgeBase{} // ensure JSON [] not null
+	}
+
+	if opts.JSONOut {
+		return format.WriteEnvelope(iostreams.IO.Out, format.Success(listResult{Items: items}, nil))
+	}
+
+	if len(items) == 0 {
+		fmt.Fprintln(iostreams.IO.Out, "(no knowledge bases)")
+		return nil
+	}
+
+	tw := tabwriter.NewWriter(iostreams.IO.Out, 0, 0, 2, ' ', 0)
+	fmt.Fprintln(tw, "ID\tNAME\tDOCS\tUPDATED")
+	now := time.Now()
+	for _, kb := range items {
+		name := text.Truncate(40, kb.Name)
+		docs := text.Pluralize(int(kb.KnowledgeCount), "doc")
+		updated := text.FuzzyAgo(now, kb.UpdatedAt)
+		fmt.Fprintf(tw, "%s\t%s\t%s\t%s\n", kb.ID, name, docs, updated)
+	}
+	return tw.Flush()
+}