docs(codex): use relative source.path inside marketplace root

Re-checked the docs PR against developers.openai.com/codex/plugins/build,
which states: "Keep source.path relative to the marketplace root, start
it with ./, and keep it inside that root."

The earlier sideload instructions used an absolute path
(/Users/YOU/src/mem0/mem0-plugin) which violates that rule. Updated:

- docs/integrations/codex.mdx Option B — clone under ~/codex-plugins/,
  use "./codex-plugins/mem0-source/mem0-plugin", restart Codex made an
  explicit step.
- mem0-plugin/README.md Option B — same fix.
- Updated the "plugin/read failed in TUI" troubleshooting entry to
  point at the relative-path requirement.

mem0-plugin/README.md

@@ -61,7 +61,13 @@ Then in Codex, browse the repo's plugin directory and install Mem0.
 
 **Option B — Personal marketplace**:
 
-Add to `~/.agents/plugins/marketplace.json`:
+Clone the repo somewhere under your home directory (Codex requires `source.path` to be relative and inside the marketplace root, which is `~/` for personal installs):
+
+```bash
+git clone https://github.com/mem0ai/mem0.git ~/codex-plugins/mem0-source
+```
+
+Then add `~/.agents/plugins/marketplace.json` with a path relative to `~/`:
 
 ```json
 {
@@ -74,7 +80,7 @@ Add to `~/.agents/plugins/marketplace.json`:
       "name": "mem0",
       "source": {
         "source": "local",
-        "path": "/path/to/mem0/mem0-plugin"
+        "path": "./codex-plugins/mem0-source/mem0-plugin"
       },
       "policy": {
         "installation": "AVAILABLE",
@@ -86,6 +92,8 @@ Add to `~/.agents/plugins/marketplace.json`:
 }
 ```
 
+Restart Codex, then run `codex /plugins` and install Mem0 from the `Mem0 Plugins` marketplace.
+
 **Option C — Direct MCP configuration** (fastest, MCP-only):
 
 Codex reads MCP servers from `~/.codex/config.toml` as TOML. Add: