docs(agents): convert install shell snippets to markdown refs

agents/install-skills.md

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+# Agent Skills Commands
+
+Reference commands for listing, installing, and updating skills via `npx skills`. Run only the
+commands you need.
+
+## Inspect and update installed skills
+
+```bash
+# list all installed skills (project + global)
+npx skills list
+
+# list only global skills
+npx skills ls -g
+
+# check for updates
+npx skills check
+
+# update all installed skills
+npx skills update
+
+# remove from global scope
+npx skills remove --global web-design-guidelines
+```
+
+## Discover skills from repositories
+
+```bash
+# list skills in a repository
+npx skills add vercel-labs/agent-skills --list
+```
+
+## Install commonly used skill packs
+
+```bash
+# superpowers
+npx skills add -g obra/superpowers --agent '*' --skill '*'
+
+# github skills
+npx skills add -g github/awesome-copilot --agent '*' --skill 'git-commit'
+
+# find skills
+npx skills add -g vercel-labs/skills --agent '*'
+
+# skill-creator
+npx skills add -g anthropics/skills --agent '*' --skill 'skill-creator'
+```
+
+## Optional packs
+
+```bash
+npx skills add -g pbakaus/impeccable --agent '*' --skill '*'
+
+npx skills add -g coreyhaines31/marketingskills --agent '*' --skill '*'
+
+npx skills add -g phuryn/pm-skills --agent '*' --skill '*'
+```
+
+References:
+
+- https://github.com/vercel-labs/skills
+- https://github.com/pbakaus/impeccable
+- https://github.com/coreyhaines31/marketingskills
+- https://github.com/phuryn/pm-skills