Token bridging — update plan CSS to use `--rf-*` tokens

Update plan rune CSS to reference --rf-* tokens (the standard refrakt token namespace) as primary values, with --plan-* as fallbacks. This allows plan styles to adapt to any installed theme's token palette while remaining functional in standalone mode.

Acceptance Criteria

Plan rune CSS references --rf-* tokens where equivalent tokens exist (colors, spacing, typography, radii)
--plan-* tokens are used as fallbacks: var(--rf-color-success, var(--plan-status-done))
Built-in tokens.css still defines --plan-* tokens (standalone mode unaffected)
Visual appearance with built-in themes is unchanged
When a --rf-* theme is active, plan runes pick up the theme's palette
Both default.css and minimal.css updated consistently

Approach

Audit plan CSS files for hardcoded values and --plan-* references that have standard --rf-* equivalents. Replace with var(--rf-*, var(--plan-*)) pattern. Focus on:

Typography: --rf-font-* → --plan-font-*
Spacing: --rf-space-* → --plan-space-*
Border radius: --rf-radius-* → --plan-radius-*
Semantic colors: --rf-color-* → --plan-status-* (where applicable)

Status-specific colors (e.g., --plan-status-in-progress) have no --rf-* equivalents and remain as-is.