Add sqlite plugin: open/query/exec/script against .db files

Adds a single-file V2 plugin that exposes eight tools under the
`sqlite` namespace (open, close, list, query, exec, script, tables,
schema) so a model can read and write SQLite databases from a session.

- OpenCode discovers it automatically from ~/.config/opencode/plugins/.
- New runtime dependency: better-sqlite3 (installed in this repo's
  node_modules so the host's module resolution picks it up).
- README documents install, copy-to-another-machine, options, and
  troubleshooting; SETUP.md is a copy-paste checklist.
- Schema/options were trimmed down while authoring: the initial draft
  registered a no-op SQLite function on every open as a 'future hook';
  removed, it was dead code.

plugins/sqlite/SETUP.md

@@ -0,0 +1,121 @@
+# sqlite — install steps (copy-paste)
+
+The plugin is a single TypeScript file. It needs two npm packages
+installed into the OpenCode config's own `node_modules` and one
+`opencode.jsonc` edit. The service must be restarted after the installs.
+
+## Prerequisites
+
+- OpenCode v2.x installed and on the PATH.
+- Node.js available (OpenCode already pulls this in).
+- On Linux: a working C toolchain (`build-essential`, `python3`, `make`
+  on Debian/Ubuntu) — only required if the prebuilt `better-sqlite3`
+  binary does not match your platform.
+
+## 1. Install the OpenCode plugin SDK at the matching version
+
+```sh
+cd ~/.config/opencode
+npm i @opencode/plugin@$(opencode --version | sed 's/^v//')
+```
+
+A stale `@opencode-ai/plugin` (V1-era name) in `node_modules` is unrelated
+to the running host. Ignore it.
+
+## 2. Install the SQLite driver
+
+```sh
+cd ~/.config/opencode
+npm i better-sqlite3
+```
+
+Tested against `better-sqlite3@13.0.3`. Versions `>= 11.0.0` expose the
+same surface and should work.
+
+## 3. Drop the plugin files in place
+
+If you are starting from the directory shipped here, just keep the
+existing `~/.config/opencode/plugins/sqlite/` directory. If you are
+copying it to another machine, copy the whole `sqlite/` directory
+(`index.ts` and these docs) into the new machine's
+`~/.config/opencode/plugins/sqlite/`.
+
+## 4. Register the plugin
+
+Edit `~/.config/opencode/opencode.jsonc`:
+
+```jsonc
+{
+  "$schema": "https://opencode.ai/config.json",
+  "plugins": [
+    "./plugins/sqlite"
+  ]
+}
+```
+
+If you already have a `plugins` array, append `"./plugins/sqlite"` rather
+than replacing it. A relative path here is resolved against the **config
+file's** directory, not the project working directory.
+
+Optional, with configuration:
+
+```jsonc
+{
+  "plugins": [
+    {
+      "package": "./plugins/sqlite",
+      "options": {
+        "maxRows": 5000,
+        "maxBytes": 10485760,
+        "defaultMode": "create",
+        "allowedRoots": ["/srv/data"]
+      }
+    }
+  ]
+}
+```
+
+## 5. Restart the service and verify
+
+```sh
+opencode service restart
+opencode plugin list
+```
+
+Expect a row whose ID is `sqlite` and whose SOURCE is `local`. If the
+plugin is not there, look at
+`~/.local/share/opencode/log/opencode.log` and filter on `sqlite`.
+
+## 6. Smoke test
+
+In a session:
+
+```text
+> open ":memory:" as alias "tmp"
+> exec "CREATE TABLE hello(id INTEGER, msg TEXT); INSERT INTO hello VALUES (1, 'hi'), (2, 'world');" on "tmp"
+> query "SELECT * FROM hello ORDER BY id" on "tmp"
+> close "tmp"
+```
+
+The first call should print a confirmation. The second should report
+`changes=2` (or similar — exact value depends on whether the CREATE
+counts). The third should print two rows.
+
+## Updating
+
+To pick up a new version of `@opencode/plugin` after upgrading OpenCode,
+re-run step 1 with the new version. To pick up a newer `better-sqlite3`,
+re-run step 2. The plugin source itself is loaded fresh on every plugin
+reload — no rebuild step.
+
+## Removing
+
+```sh
+# 1. Drop the entry from opencode.jsonc
+# 2. Remove the directory
+rm -rf ~/.config/opencode/plugins/sqlite
+# 3. (Optional) drop the dependency
+cd ~/.config/opencode && npm rm better-sqlite3
+# 4. Restart so the running service stops calling the tools
+opencode service restart
+```