Add `--preview` flag as alias for `-a untrusted` to improve discoverability

What feature would you like to see?

Problem

The -a untrusted (or --ask-for-approval=untrusted) flag enables a valuable diff preview + approval workflow, but users don't easily discover it because:

• The flag name doesn't clearly communicate "show me what will change before applying"
• Users coming from tools like Aider/Cursor expect a --preview or --diff flag
• Documentation doesn't prominently feature this as a safety/review workflow

Proposed Solution

Add a --preview flag as a CLI alias that internally sets approval_policy = untrusted.

Implementation location (based on codebase inspection):

// In tui/src/cli.rs (alongside existing flags)
#[arg(long = "preview", conflicts_with = "approval_policy")]
pub preview: bool,

Then map it during config resolution in tui/src/lib.rs (similar to how --full-auto works around line 96).

Benefits

• Makes existing functionality discoverable through intuitive naming
• Zero new behavior—just a clearer entry point
• ~10 lines of code
• Aligns with user expectations from similar tools

Alternative

If adding an alias isn't desirable, a documentation PR highlighting the "preview before apply" workflow with -a untrusted would also help.

Example Usage

Current (works but not obvious):

codex -a untrusted

Proposed (clearer intent):

codex --preview

Are you interested in implementing this feature?

Yes, I can submit a PR if this approach is approved. I've already inspected the codebase and identified the integration points in tui/src/cli.rs and tui/src/lib.rs.

Additional information

_No response_