Keep the personal half of the desktop in one place, and ask before installing it

Agent instructions, skills, SSH host aliases and expansion triggers are worth
having identical on every machine one person owns, and belong in none of the
shared configuration. They live in user/ now, with a manifest saying where each
piece goes and a link-user stage that puts it there.

That stage does nothing unless the machine said yes. Somebody who clones Panama
to try the desktop keeps their own ~/.claude/CLAUDE.md exactly where it was;
the question names the destinations and defaults to no. Anything displaced goes
to config/old rather than being deleted.

~/.claude/CLAUDE.md and ~/.codex/AGENTS.md were byte-identical copies of one
file, which is the drift this exists to prevent.

Also adds the vitals toggles for the battery and Claude usage readouts, which
had preferences and no way to reach them.

user/agents/skills/slidev/references/syntax-block-frontmatter.md

@@ -0,0 +1,39 @@
+---
+name: block-frontmatter
+description: Using YAML code blocks as slide frontmatter for syntax highlighting
+---
+
+# Block Frontmatter
+
+Use a YAML code block as the frontmatter for slides when you need syntax highlighting and formatter support.
+
+## Usage
+
+Instead of traditional frontmatter `---`, use a yaml code block at the start of a slide:
+
+````md
+---
+theme: default
+---
+
+# Slide 1
+
+---
+
+```yaml
+layout: quote
+```
+
+# Slide 2
+
+---
+
+# Slide 3
+````
+
+## Key Points
+
+- Works for per-slide frontmatter only
+- Cannot use for headmatter (first frontmatter of the deck)
+- Provides syntax highlighting in editors
+- Compatible with prettier-plugin-slidev