Add start-issue cmux skill

[dapi]

Summary

• add the start-issue-cmux agent skill under skills/start-issue-cmux/
• add a safe launcher for creating a dedicated cmux workspace and running start-issue
• document lifecycle, resume/monitoring, layout guidance, and safety boundaries
• provide OpenAI skill metadata for local/global installation

Design

The skill is intentionally a thin adapter:

• start-issue remains the owner of issue resolution, branch/worktree lifecycle, init hooks, prompt rendering, and agent launch
• cmux owns workspace and surface topology
• the bundled launcher defaults to Codex, preserves the current workspace focus, honors .envrc through direnv exec ., and normalizes --cwd

It does not perform merge, release, deploy, production mutation, or external messaging.

Validation

• skill-creator quick validator: passed
• launcher bash -n: passed
• launcher --help and invalid-directory checks: passed
• stub cmux integration test: passed
• direnv exec . mise exec go@1.24 -- make test: passed