docs: 第二层改名 Cold Context，清理四组已不存在的符号名

## 问题

README 与两份 ARCHITECTURE / OVERVIEW 仍在用旧称「Document 层」，
而第二层早已改名 cold context（工具名 `coldcontext_query` / `coldcontext_commit`）。

不止是层名。文档里引用的四个符号在代码里**一个都不存在**：

| 文档里写的 | 代码里实际 |
|---|---|
| `docStore.ContextToDoc` | `coldcontextStore.ContextToColdContext` |
| `FindColdDocs` | `FindColdContexts` |
| `docToTriples` | `coldDocToTriples` |
| `docStore.SetVectorizer` | （已无此调用，embedder 构造期注入） |

四处都用 `grep -rn "\b名字\b" --include=*.go` 确认过：旧名 0 处，新名各自 4 处。

## 改了什么

- `assets/docs/zh/ARCHITECTURE.md` 13 处、`en/ARCHITECTURE.md` 15 处
- `assets/docs/zh|en/OVERVIEW.md` 各 3 处
- README 两版：「文档层」→「冷上下文层」
- `assets/branding/mascot-xiaozhai-guide.md` 的丝带配色表（三色分别对应三层）

## 顺带修正层号

ARCHITECTURE 用的是 **L1 Context / L2 Cold Context / L3 Stable**，
我上一版 README 写成了 L0/L1/L2 —— 少了一层，且与架构文档对不上。已改为 L1/L2/L3。

## 顺带修掉断掉的交叉引用

`internal/agent/core/toolcall.go` 与 `recallcoldcontext_test.go` 的注释写
「ARCHITECTURE「记忆流转」② Document 层的读取分支」，指的正是刚改名的节。
不改就成了悬空引用。三个测试函数名（`TestContextToDoc` 等）指向已删函数，一并改名。

## 验证

- 全仓 `grep` 旧名：`*.md` / `*.go` 均 0 处（排除 build/third_party/.pi）
- `go build ./...` ✓
- `go test ./internal/memory/coldcontext/ ./internal/agent/core/` ✓

assets/docs/en/OVERVIEW.md

@@ -29,7 +29,7 @@ require touching the kernel, and no second version of business truth accumulates
 
 **Three-Layer Memory Architecture** — Manages information retention across long agent runtimes through a tiered storage strategy:
 - **Context Layer**: In-memory pretrained word embedding scored event window (StaticEmbedder word vectors → CosineSimilarity, TF-IDF fallback), maintains recent context in real-time, low-relevance events automatically sink to the next layer
-- **Document Layer**: JSON files + TF-IDF vector-indexed temporary memory, supports explicit submission and implicit archival, cold data distills to Graph
+- **Cold Context Layer (L2)**: JSON-file-backed cold memory with dense + TF-IDF retrieval, supports explicit submission and implicit archival, cold data distills to Graph
 - **Stablemem Layer**: SQLite graph database, persists entities and relations, BFS traversal recall, distillation pipeline extracts triples from conversations
 
 Three progressive layers — context, cold archive, long-term stable memory — form an information decay and consolidation pipeline from short-term to persistent storage.
@@ -41,7 +41,7 @@ Three progressive layers — context, cold archive, long-term stable memory —
 - **Blocks carry their own multimodal vector and fingerprint**, and are retrieved directly — there is no
   media marker in the body text any more.
 - **No separate GC, no reference counting, no keep-set**: blocks follow the single-layer invariant —
-  Context → Document → Graph is a **migration** of a block, not a copy, and nothing is kept alive by
+  Context → Cold Context → Graph is a **migration** of a block, not a copy, and nothing is kept alive by
   references; **deleting a block deletes its content.**
 - **Reaches the plugin boundary since v1.1.1**: plugins read and write media through `InsertWithMedia` /
   `InjectInputMedia`; the model attaches media via the `media_digests` argument of `memory_commit` / `coldcontext_commit`.
@@ -64,7 +64,7 @@ Code is in the project root, implemented in Go.
 
 **Memory System** (`internal/memory/`):
 - **GraphDB** (`graph.go`) — SQLite, entities + relations tables, BFS traversal
-- **Document Store** (`document/document.go`) — Temporary memory, JSON files + TF-IDF vector index, consume-on-read
+- **Cold Context Store** (`internal/memory/coldcontext/`) — cold memory with two-phase retrieval (Peek, then Remove)
 - **Text Memory** (`text/text.go`) — Raw conversation logs, JSONL file rotation
 - **Social Store** (`social/social.go`) — Persona traits + relationship network, wraps GraphDB
 - **Memory Indexer** (`indexer.go`) — Auto-vectorizes GraphDB entities, recalls and injects into system prompt on user input