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.

mem0-plugin/README.md

@@ -86,25 +86,17 @@ Add to `~/.agents/plugins/marketplace.json`:
 }
 ```
 
-**Option C — Manual MCP configuration**:
+**Option C — Direct MCP configuration** (fastest, MCP-only):
 
-Add to your Codex MCP config:
+Codex reads MCP servers from `~/.codex/config.toml` as TOML. Add:
 
-```json
-{
-  "mcpServers": {
-    "mem0": {
-      "type": "http",
-      "url": "https://mcp.mem0.ai/mcp/",
-      "headers": {
-        "Authorization": "Token ${MEM0_API_KEY}"
-      }
-    }
-  }
-}
+```toml
+[mcp_servers.mem0]
+url = "https://mcp.mem0.ai/mcp"
+bearer_token_env_var = "MEM0_API_KEY"
 ```
 
-This installs the MCP server and the Mem0 SDK skill. Codex uses the skill-based memory protocol instead of lifecycle hooks.
+Export `MEM0_API_KEY` in your shell and restart Codex. This gives you the MCP tools without the plugin skills or lifecycle hooks. `codex mcp add` only supports stdio servers, so HTTP servers like Mem0's must be added via `config.toml` directly (or via the **Plugins → Connect to a custom MCP → Streamable HTTP** UI in the Codex app).
 
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