docs: address review feedback on pre-existing issues

- Fix duplicate copilot example in README.md (replace with codex)
- Fix invalid gemini --integration-options="--skills" example (gemini
  does not support --skills)
- Update generic integration comment from 'Unsupported agent' to
  'Bring your own agent; requires --commands-dir'
- Clarify EXTENSION-USER-GUIDE.md: skills auto-register for
  skills-based integrations, not only with --integration-options

Author: mnriem

README.md

@@ -480,7 +480,7 @@ You will be prompted to select the AI agent you are using. You can also proactiv
 ```bash
 specify init <project_name> --integration copilot
 specify init <project_name> --integration gemini
-specify init <project_name> --integration copilot
+specify init <project_name> --integration codex
 
 # Or in current directory:
 specify init . --integration copilot

extensions/EXTENSION-USER-GUIDE.md

@@ -189,7 +189,7 @@ Provided commands:
 
 ### Automatic Agent Skill Registration
 
-If your project was initialized with `--integration-options="--skills"`, extension commands are **automatically registered as agent skills** during installation. This ensures that extensions are discoverable by agents that use the [agentskills.io](https://agentskills.io) skill specification.
+If your project uses a skills-based integration (e.g., `--integration claude`, `--integration codex`) or was initialized with `--integration-options="--skills"`, extension commands are **automatically registered as agent skills** during installation. This ensures that extensions are discoverable by agents that use the [agentskills.io](https://agentskills.io) skill specification.
 
 ```text
 ✓ Extension installed successfully!

src/specify_cli/__init__.py

@@ -1016,8 +1016,8 @@ def init(
         specify init --here
         specify init --here --force  # Skip confirmation when current directory not empty
         specify init my-project --integration claude   # Claude installs skills by default
-        specify init --here --integration gemini --integration-options="--skills"
-        specify init my-project --integration generic --integration-options="--commands-dir .myagent/commands/"  # Unsupported agent
+        specify init --here --integration gemini
+        specify init my-project --integration generic --integration-options="--commands-dir .myagent/commands/"  # Bring your own agent; requires --commands-dir
         specify init my-project --offline  # Use bundled assets (no network access)
         specify init my-project --integration claude --preset healthcare-compliance  # With preset
     """