Add memory plugin: three-tier markdown store with plugin-driven retrieval

A durable memory layer for OpenCode. Structure:

  ROOT.md          total plan: goals, hard rules, directory registry,
                   and a scenario -> file routing table
  <category>/
    index.md       one line per file, COMPILED from frontmatter
    <slug>.md      the memory itself

Two roots, same shape: <config>/memory/ (global) and
<project canonical>/.memory/ (project). Project wins on conflict; global
hard rules stay resident everywhere.

Why a plugin rather than a skill that tells the model to read a folder:
model-driven retrieval is unreliable — the model forgets, reads half the
files, or reads the wrong ones. Markdown is the human-facing format; the
plugin is the engine that parses, scores, and injects.

Measured, not assumed: FTS5's two built-in tokenizers both fail on this
corpus. On SQLite 3.53.4 with "部署脚本用 pnpm 而不是 npm...": unicode61
emits 7 terms for the whole sentence and matches 0 hits for "部署";
trigram needs >= 3 characters and also returns 0 for that two-character
word. So tokenization happens in the plugin (CJK bigrams, latin words)
and no SQL index is used — a thousand memories are fine to scan.

index.md and ROOT's two tables are compiled, never hand-edited: a
hand-maintained index drifts, a generated one cannot. Hit counters live
in .state.json so cross-device sync does not conflict on every recall.

Tools are named recall/remember/forget/inspect rather than
write/read/status: generic names were observed shadowing the harness's
own built-in tools.

memory/ROOT.md

@@ -0,0 +1,41 @@
+# 记忆总纲 — 全局
+
+> 跨项目通用记忆：你的偏好、技术决策、通用约定、踩过的坑。
+> 由 memory 插件维护。`auto:begin` / `auto:end` 之间的表格是编译产物，不要手改；
+> 其余区域随便写。
+
+## 当前目标
+
+<!-- 手写：现在在做什么。插件不会碰这里。 -->
+
+## 铁规则（常驻，永不衰减）
+
+<!-- 手写：只写硬性要求。rules/ 目录下的文件会被自动常驻注入。 -->
+
+## 目录登记
+
+<!-- auto:begin registry
+
+| 目录 | 概述 | 条目 |
+|---|---|---|
+| decisions/ | 选 Markdown 因为人类可纠正性和可同步性，检索由插件做而非模型。 | 1 |
+| pitfalls/ | unicode61 把整段中文当一个 token，trigram 漏掉两字词，必 | 1 |
+| rules/ | 不可违反的硬规则，常驻 | 1 (1 钉住) |
+<!-- auto:end
+
+## 加载路由表
+
+<!-- auto:begin routing
+
+| 场景 / 触发 | 读哪个文件 | 类型 |
+|---|---|---|
+| FTS5 | pitfalls/sqlite-cjk-tokenizer.md | fact |
+| memory | decisions/memory-store-format.md | fact |
+| SQLite | pitfalls/sqlite-cjk-tokenizer.md | fact |
+| 中文检索 | pitfalls/sqlite-cjk-tokenizer.md | fact |
+| 为什么用 markdown | decisions/memory-store-format.md | fact |
+| 全文检索 | pitfalls/sqlite-cjk-tokenizer.md | fact |
+| 分词 | pitfalls/sqlite-cjk-tokenizer.md | fact |
+| 存储选型 | decisions/memory-store-format.md | fact |
+| 记忆 | decisions/memory-store-format.md | fact |
+<!-- auto:end