Register the kstack retro skill for Copilot

[chokevin]

Problem

Invoking kstack-retro through Copilot's skill loader reports that the skill is unavailable, even though the implementation exists at skills/retro/SKILL.md. This forces agents to locate and read the skill manually, defeating discoverability and making the documented /kstack-retro workflow unreliable.

Expected behavior

The retro workflow should be invokable consistently from Copilot using the documented name. Either register/symlink kstack-retro, or standardize the documented invocation on the actually registered name and ensure it appears in the available-skills list.

Acceptance criteria

• A fresh Copilot session can invoke the retro skill without manually locating SKILL.md.
• The global kstack instructions and skill trigger use the same name.
• The registration path is covered by the existing skill installation/update process.