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/decisions/index.md

@@ -0,0 +1,8 @@
+# decisions 索引
+
+<!-- auto:begin decisions
+
+| 文件 | 概述 | 重点 |
+|---|---|---|
+| memory-store-format.md | 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。 | 纯文本可手改、可 git diff；SQLite 的相关性排序解决的是规模问题，而记忆只有千级 |
+<!-- auto:end