Add -- passthrough args to cmux new and cmux start

[zandaleph]

Summary

• Adds -- separator support to cmux new and cmux start so that any arguments after -- are forwarded directly to the claude invocation
• Enables use cases like --dangerously-skip-permissions and other claude CLI flags that aren't exposed by cmux today
• Updates help text for both commands with examples

Usage

# Launch with dangerous permissions bypass
cmux start iron -- --dangerously-skip-permissions

# Combine with existing -p flag
cmux new my-feature -p "implement the thing" -- --dangerously-skip-permissions

# Multiple claude args work too
cmux start iron -- --dangerously-skip-permissions --no-update-notifier

Motivation

When migrating from tools like gastown that pass extra flags to claude on startup, there was no way to forward arbitrary claude CLI flags through cmux. The -- convention is a standard Unix idiom for separating a wrapper's own arguments from those passed to a subprocess, making it guessable and consistent with tools like npm run, git, etc.

Test plan

• cmux new --help and cmux start --help show updated usage with [-- <claude-args>]
• cmux new <branch> (no --) still works as before
• cmux start <branch> (no --) still works as before
• cmux start <branch> -- --dangerously-skip-permissions passes the flag to claude
• cmux new <branch> -p "prompt" -- --flag passes both prompt and flag correctly
• cmux new -- --flag (no branch) shows usage error

🤖 Generated with Claude Code

[zandaleph]

Closing in favor of #11 which covers the same ground. I implemented this independently and arrived at the same approach — can confirm it works well. One small suggestion for #11: adding an example line to the help text would help with discoverability, e.g. Example: cmux start my-branch -- --dangerously-skip-permissions.