docs(codex): lead sideload with CLI, flag auto-MCP, align server name

Rework the Codex install flow around `codex plugin marketplace add
<clone>` so users can lean on the repo's bundled
`.agents/plugins/marketplace.json` instead of hand-authoring one.
This removes the "path must be under ~/" constraint that was tripping
people up.

Also:
- Call out that sideloading auto-registers `mem0` via .codex-mcp.json,
  so Option A (Direct MCP) and Option B (sideload) must not be combined.
- Rename the Direct MCP snippet on the platform page from `mem0-mcp`
  to `mem0` to match the bundled plugin — prevents silent duplicate
  servers for users who follow one path then try the other.
- Drop the trailing slash in .codex-mcp.json's URL to match the rest
  of the docs.
- Add `codex plugin marketplace upgrade` / `remove` and the plugin
  cache path (~/.codex/plugins/cache/...).
- New troubleshooting entries for duplicate MCP registration and for
  hooks breaking after a clone is moved (the installer bakes absolute
  paths, so moving the clone requires re-running it).

mem0-plugin/.codex-mcp.json

@@ -1,7 +1,7 @@
 {
   "mcpServers": {
     "mem0": {
-      "url": "https://mcp.mem0.ai/mcp/",
+      "url": "https://mcp.mem0.ai/mcp",
       "bearer_token_env_var": "MEM0_API_KEY"
     }
   }