docs: add @latest tag and cache clearing instructions (griffinmartin#180)

## Summary

- Updates all plugin references in README.md and installation.md from
`opencode-claude-auth` to `opencode-claude-auth@latest` so OpenCode
always pulls the newest version on startup
- Adds a troubleshooting row for "Plugin not updating to latest version"
with the targeted cache delete command (`rm -rf
~/.cache/opencode/packages/opencode-claude-auth@latest/`)
- Adds an "Upgrading" section to installation.md for users migrating
from a pinned version

These changes address recurring reports from griffinmartin#145 where users were stuck
on cached v1.4.7 after v1.4.9 was released.

Author: griffinmartin

README.md

@@ -37,11 +37,11 @@ Install the opencode-claude-auth plugin and configure it by following: https://r
 
    ```json
    {
-     "plugin": ["opencode-claude-auth"]
+     "plugin": ["opencode-claude-auth@latest"]
    }
    ```
 
-   > No manual `npm install` is needed — OpenCode [automatically installs npm plugins using Bun at startup](https://opencode.ai/docs/plugins/#how-plugins-are-installed).
+   > The `@latest` tag ensures OpenCode always pulls the newest version on startup. No manual `npm install` is needed — OpenCode [automatically installs npm plugins using Bun at startup](https://opencode.ai/docs/plugins/#how-plugins-are-installed).
 
 2. **Use it** — just run OpenCode. The plugin handles auth automatically using your Claude Code credentials.
 
@@ -111,6 +111,7 @@ If only one account is found, the switcher is hidden and the plugin uses it dire
 | Keychain read timed out                             | Restart Keychain Access (can happen on macOS Tahoe)                                                                |
 | "Credentials are unavailable or expired"            | Run `claude` to refresh your Claude Code credentials                                                               |
 | "Extra usage is required for long context requests" | Your conversation exceeded 200k tokens. See [Long context (1M)](#long-context-1m) below                            |
+| Plugin not updating to latest version               | Delete the cached package: `rm -rf ~/.cache/opencode/packages/opencode-claude-auth@latest/` then restart OpenCode  |
 
 ### Diagnostic logging
 
@@ -146,7 +147,7 @@ Add `enable1mContext` to any agent in your `opencode.json` (project-level or `~/
 
 ```json
 {
-  "plugin": ["opencode-claude-auth"],
+  "plugin": ["opencode-claude-auth@latest"],
   "agent": {
     "build": {
       "enable1mContext": true

installation.md

@@ -46,11 +46,11 @@ This will prompt you to log in and store credentials in Keychain (macOS) or `~/.
 
 Edit the OpenCode configuration file at `~/.config/opencode/opencode.json`.
 
-Add `opencode-claude-auth` to the `plugin` array:
+Add `opencode-claude-auth@latest` to the `plugin` array:
 
 ```json
 {
-  "plugin": ["opencode-claude-auth"]
+  "plugin": ["opencode-claude-auth@latest"]
 }
 ```
 
@@ -60,14 +60,14 @@ Or run this command to do it automatically:
 node -e "
 const fs = require('fs'), p = require('path').join(require('os').homedir(), '.config/opencode/opencode.json');
 const c = fs.existsSync(p) ? JSON.parse(fs.readFileSync(p,'utf8')) : {};
-c.plugin = [...new Set([...(Array.isArray(c.plugin) ? c.plugin : []), 'opencode-claude-auth'])];
+c.plugin = [...new Set([...(Array.isArray(c.plugin) ? c.plugin : []), 'opencode-claude-auth@latest'])];
 fs.mkdirSync(require('path').dirname(p), {recursive:true});
 fs.writeFileSync(p, JSON.stringify(c, null, 2));
-console.log('Added opencode-claude-auth to', p);
+console.log('Added opencode-claude-auth@latest to', p);
 "
 ```
 
-No manual `npm install` is needed — OpenCode [automatically installs npm plugins using Bun at startup](https://opencode.ai/docs/plugins/#how-plugins-are-installed).
+The `@latest` tag ensures OpenCode always pulls the newest version on startup. No manual `npm install` is needed — OpenCode [automatically installs npm plugins using Bun at startup](https://opencode.ai/docs/plugins/#how-plugins-are-installed).
 
 ### Step 2: Verification
 
@@ -77,7 +77,17 @@ Verify the plugin was added:
 cat ~/.config/opencode/opencode.json
 ```
 
-You should see `opencode-claude-auth` in the `plugin` array.
+You should see `opencode-claude-auth@latest` in the `plugin` array.
+
+## Upgrading
+
+If you previously installed `opencode-claude-auth` without the `@latest` tag, update your config to use `opencode-claude-auth@latest` as shown above.
+
+If the plugin isn't picking up a new version, clear the cached package and restart OpenCode:
+
+```bash
+rm -rf ~/.cache/opencode/packages/opencode-claude-auth@latest/
+```
 
 ## Done