add skill-installer skill

.github/skills/skill-installer/SKILL.md

@@ -0,0 +1,54 @@
+---
+name: skill-installer
+description: >
+  Install a skill into this repository or into the global Copilot skills
+  directory. Defaults to repo-only and prompts for source, scope, and overwrite
+  behavior.
+user-invocable: true
+---
+
+# Skill Installer
+
+Use this skill to install another skill from a local path, a directory, or a GitHub `SKILL.md` URL.
+
+## Default behavior
+
+- Target scope defaults to `repo`
+- The installer asks for missing details interactively
+- Existing files are never overwritten without confirmation
+- If the source is a local directory, you can choose whether to copy supporting files too
+
+## Supported inputs
+
+- Local `SKILL.md` file
+- Local skill directory
+- GitHub blob URL to a `SKILL.md` file
+
+## Install script
+
+Run:
+
+```bash
+bash .github/skills/skill-installer/scripts/install-skill.sh
+```
+
+Or provide a source directly:
+
+```bash
+bash .github/skills/skill-installer/scripts/install-skill.sh https://github.com/casey/just/blob/master/skills/just/SKILL.md
+```
+
+The script installs to:
+
+- `.github/skills/<skill-name>/` for repo-only installs
+- `~/.copilot/skills/<skill-name>/` for global installs
+
+## What it asks
+
+The installer prompts for:
+
+1. Source path or URL
+2. Target scope: repo or global
+3. Skill name if it cannot be inferred
+4. Whether to copy supporting files
+5. Whether to overwrite an existing install