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.

.gitignore

@@ -5,6 +5,7 @@
 #
 #   ~/.config/opencode/skills/     -> agent skills
 #   ~/.config/opencode/plugins/   -> server plugins (auto-discovered)
+#   ~/.config/opencode/memory/    -> memory store written by the memory plugin
 #
 # A .git directory inside either of those gets picked up as a plugin and fails
 # to load, so the repository lives one level up, here, and everything that is
@@ -17,8 +18,14 @@
 /*
 !/skills/
 !/plugins/
+!/memory/
 !/.gitignore
 
+# Per-memory hit counters. Derived data: regenerating it costs one recall, and
+# committing it would rewrite the file on every retrieval and create merge
+# conflicts across devices.
+memory/**/.state.json
+
 # UTF-8 shell launcher (see https://www.xsinfo.vip/windows/01-opencode-utf8.html).
 # Portable: it only calls chcp and pwsh.exe from PATH, so it is safe to share.
 !/pwsh.cmd