[docs] Update codex installation docs for clarity

[bdotdub]

Summary

The codex installation docs were slightly confusing when I went through them. We're not actually storing the PAT in the config. As well, storing the PAT as an env var is a requirement so installation shouldn't be collapsed, where it is easily missed.

What changed

• Updated installation docs

MCP impact

• No tool or API changes
• Tool schema or behavior changed
• New tool added

Security / limits

• No security or limits impact
• Auth / permissions considered
• Data exposure, filtering, or token/size limits considered

Tool renaming

• I am renaming tools as part of this PR (e.g. a part of a consolidation effort)
  • I have added the new tool aliases in deprecated_tool_aliases.go
• I am not renaming tools as part of this PR

Lint & tests

• Linted locally with ./script/lint
• Tested locally with ./script/test

Docs

• Not needed
• Updated (README / docs / examples)

[Copilot]

Pull request overview

This PR improves the clarity of the Codex installation documentation by removing a confusing comment that implied the PAT should be stored in the config file (when it's actually stored as an environment variable), and by making the PAT security instructions more prominent by converting them from a collapsible section to a standard heading.

Key changes:

• Removed misleading comment about replacing PAT in config file
• Promoted PAT security instructions from collapsible details to a visible section heading