feat(commands): add slash command for openspec-ff-change skill

[raphaelbahat]

Is your feature request related to a problem? Please describe.

Currently, the openspec-ff-change skill is available to fast-forward artifact creation, but there is no corresponding /opsx-* slash command / prompt template available in the repository.

OpenSpec maintains a 1:1 symmetry across workflows where openspec-* skills (such as openspec-apply-change, openspec-new-change, and openspec-propose) have matching opsx-* prompt commands. The absence of an opsx-ff-change command breaks this symmetry and makes it less convenient to invoke the fast-forward workflow directly via prompt templates.

Describe the solution you'd like

Add a slash command / prompt template corresponding to the openspec-ff-change skill.

For example:

• Command Name: opsx-ff-change (or opsx-ff)
• Associated Skill: openspec-ff-change
• Description: Fast-forward through OpenSpec artifact creation to generate all required implementation artifacts in a single pass.

Additional Context

The openspec-ff-change skill definition already mentions trigger phrases like "openspec ff" and "opsx ff". Providing an official command template will align the command interface with the rest of the openspec skill suite.