feat: add code as a shortcut for harness open - #211

Add spinloop code, a one-word top-level shortcut for spinloop harness open.

Summary

• New spinloop code command: the same launch as spinloop harness open — the same flags, Spinloop application, --env configuration, fleet routing, and trailing-argument forwarding
• Factored the launch out of openCmd() into a shared openLaunchCmd builder in a new cmd/spinloop/code.go, so harness open and code are one implementation and cannot drift
• Documented the shortcut: a new docs/commands/code.md, a command-index entry, cross-links from harness.md and the getting-started guide, and a one-line mention in the README
• Specified the behaviour in a new code-command capability (OpenSpec change), mirroring up-command

Implementation details

code is a shortcut, not a second command: both spellings are built by the same openLaunchCmd(use, short, long), so the flag set, the --spinloop/-O handling, --env, fleet routing, and forwarding are shared by construction. The tests pin this two ways — a parity test that feeds identical arguments to both spellings and asserts they reach the harness the same way (and fail the same way), and a root-dispatch test that code routes to the launch. Shell completion needed no changes: the surface is derived from the command tree, so code is offered and its first slot completes Spinloops automatically.

Closes #203