Feat v0.6.13 responsive spacing

Add spacing token scale (--sp-1 through --sp-12, 4px grid) and tablet
breakpoint (max-width: 1024px). Migrate all kernel CSS and 12 in-tree
extension packages from hardcoded padding/margin/gap to design tokens.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/EXTENSION-CSS.md

@@ -1,6 +1,6 @@
 # Extension CSS Contract
 
-> **Version**: v0.6.12 — Extension CSS Isolation
+> **Version**: v0.6.13 — Responsive & Spacing
 
 This document defines the CSS contract between the Armature kernel and extension
 packages. Extensions **must** follow these rules; the kernel guarantees the listed
@@ -113,6 +113,34 @@ instead of hardcoded colors to respect the user's theme.
 | `--purple` | Purple accent |
 | `--purple-dim` | Dimmed purple background |
 
+### Spacing
+
+Use spacing tokens instead of hardcoded values for padding, margin, and gap.
+For sub-4px values (1px, 2px, 3px) used in borders and fine detail, hardcoded
+values are acceptable.
+
+| Variable | Value | Computed |
+|----------|-------|---------|
+| `--sp-1` | `0.25rem` | 4px |
+| `--sp-2` | `0.5rem` | 8px |
+| `--sp-3` | `0.75rem` | 12px |
+| `--sp-4` | `1rem` | 16px |
+| `--sp-5` | `1.25rem` | 20px |
+| `--sp-6` | `1.5rem` | 24px |
+| `--sp-8` | `2rem` | 32px |
+| `--sp-10` | `2.5rem` | 40px |
+| `--sp-12` | `3rem` | 48px |
+
+Example:
+
+```css
+.ext-my-app-card {
+    padding: var(--sp-3) var(--sp-4);   /* 12px 16px */
+    gap: var(--sp-2);                    /* 8px */
+    margin-bottom: var(--sp-4);          /* 16px */
+}
+```
+
 ### Layout & Typography
 
 | Variable | Purpose |
@@ -130,6 +158,22 @@ instead of hardcoded colors to respect the user's theme.
 
 ---
 
+## Responsive Breakpoints
+
+The kernel uses these standard breakpoints. Extensions should use the same
+values for consistency.
+
+| Name | Media Query | Use Case |
+|------|-------------|----------|
+| Mobile | `@media (max-width: 768px)` | Phone-sized, single column |
+| Tablet | `@media (max-width: 1024px)` | Tablet/small laptop, narrower sidebars |
+| Desktop | Default (no query) | Full layout |
+
+CSS custom properties cannot be used in `@media` queries — use the pixel
+values directly.
+
+---
+
 ## What Is Internal
 
 Everything not listed above is **internal kernel CSS** and may change between