feat(cli): chunk subtree + MCP chunk_list tool + curation rationale

New subtree (chunk list / view / delete) exposes RAG retrieval
debugging primitives with SDK-grounded field set (23 Chunk fields).
Pagination follows v0.4 canon: --limit / --page-size (1..1000) /
--all-pages.

- chunk list --doc <id>: enumerate by ChunkIndex (separate from
  search chunks which is hybrid retrieval; Long help documents the
  distinction)
- chunk view <id>: scope-less render via /chunks/by-id route; full
  content verbatim
- chunk delete <id> --doc <id>: scope-flag + scope-id; L-13
  destructive; 404 NOT idempotent; resource.not_found /
  auth.forbidden / input.confirmation_required typed exit codes
  documented in Long help

MCP server gains chunk_list as 10th curated tool. Schema deliberately
exposes only doc_id + limit (no pagination workflow on MCP); response
includes truncated_at_limit flag when total > limit.

cli/AGENTS.md MCP curation rationale rewritten: curated read-only is
a deliberate product call because the server side does not yet
enforce per-token scope. When server scope ships, mutation tools can
land in the MCP surface.

Shared helper cli/internal/text/timeago_string.go (FuzzyAgoStr)
extracted from session list during the C2 quality-review pass.

cli/cmd/root.go

@@ -11,6 +11,7 @@ import (
 	apicmd "github.com/Tencent/WeKnora/cli/cmd/api"
 	"github.com/Tencent/WeKnora/cli/cmd/auth"
 	chatcmd "github.com/Tencent/WeKnora/cli/cmd/chat"
+	chunkcmd "github.com/Tencent/WeKnora/cli/cmd/chunk"
 	contextcmd "github.com/Tencent/WeKnora/cli/cmd/context"
 	"github.com/Tencent/WeKnora/cli/cmd/doc"
 	"github.com/Tencent/WeKnora/cli/cmd/doctor"
@@ -130,6 +131,7 @@ hybrid searches against a WeKnora server from your shell or an AI agent.`,
 	cmd.AddCommand(chatcmd.NewCmd(f))
 	cmd.AddCommand(sessioncmd.NewCmd(f))
 	cmd.AddCommand(agentcmd.NewCmd(f))
+	cmd.AddCommand(chunkcmd.NewCmdChunk(f))
 	cmd.AddCommand(mcpcmd.NewCmd(f))
 	return cmd
 }