docs(codex): fix broken install instructions, lead with direct MCP

The support ticket that surfaced this found three overlapping issues:

1. docs/integrations/codex.mdx shipped a marketplace.json snippet with
   path "./plugins/mem0" — a directory that does not exist, with no clone
   prerequisite documented, and using a folder name that does not match
   the actual mem0-plugin/ directory. Users copy-pasted it verbatim and
   hit "plugin/read failed in TUI".

2. The "Manual MCP Configuration" option used a JSON mcpServers block.
   Codex reads MCP servers as TOML in ~/.codex/config.toml, not JSON.
   Same bug in docs/platform/mem0-mcp.mdx (no Codex accordion at all on
   main) and mem0-plugin/README.md Option C.

3. The page claimed "Codex uses a skill-based approach instead of
   lifecycle hooks" — stale; hooks are now available via opt-in
   installer (mem0-plugin/scripts/install_codex_hooks.py).

Lead with the working TOML MCP config (zero dependencies, works today),
demote the marketplace.json to a "Sideload (Advanced)" section with the
required git clone step and the correct mem0-plugin path, and point
sideloaders at the hooks installer + codex_hooks feature flag. Added
troubleshooting entries for the TUI read error and hooks-not-firing.

docs/platform/mem0-mcp.mdx

@@ -10,7 +10,7 @@ estimatedTime: "~2 minutes"
   - Mem0 Platform account (<a href="https://app.mem0.ai?utm_source=oss&utm_medium=platform-mem0-mcp" rel="nofollow">Sign up here</a>)
   - API key (<a href="https://app.mem0.ai/settings/api-keys?utm_source=oss&utm_medium=platform-mem0-mcp" rel="nofollow">Get one from dashboard</a>)
   - Node.js 14+ (for npx)
-  - An MCP-compatible client (Claude, Claude Code, Cursor, Windsurf, VS Code, OpenCode)
+  - An MCP-compatible client (Claude, Claude Code, Codex, Cursor, Windsurf, VS Code, OpenCode)
 </Info>
 
 ## What is Mem0 MCP?
@@ -86,6 +86,20 @@ You can also configure individual clients:
     ```
   </Accordion>
 
+  <Accordion title="Codex">
+    Codex reads MCP servers from `~/.codex/config.toml` as TOML (not JSON). Add:
+
+    ```toml
+    [mcp_servers.mem0-mcp]
+    url = "https://mcp.mem0.ai/mcp"
+    bearer_token_env_var = "MEM0_API_KEY"
+    ```
+
+    Export `MEM0_API_KEY` in the shell you launch Codex from, then restart Codex. `codex mcp add` only supports stdio servers, so HTTP servers must be added via `config.toml` directly — or via the **Plugins → Connect to a custom MCP → Streamable HTTP** UI in the Codex app.
+
+    For the full plugin experience (memory protocol skill + opt-in lifecycle hooks), see the [Codex integration guide](/integrations/codex).
+  </Accordion>
+
   <Accordion title="Cursor">
     ```bash
     npx mcp-add \