fix(site): standardize PR status icon colors to match diff colors

[DanielleMaywood]

Summary

The git PR status icons in the agents sidebar used ad-hoc color values
(text-green-500, text-purple-500, text-content-destructive) that
didn't match the diff file-tree indicators. This PR standardizes all
diff-related colors behind new semantic Tailwind tokens and CSS
variables, following the existing --content-* / --surface-* pattern.

New design tokens

Added to index.css (light + dark) and wired through tailwind.config.js:

Token	Light	Dark	Tailwind class	Usage
--git-added	142 72% 29%	142 77% 73%	text-git-added	File tree labels, PR open icon
--git-deleted	0 74% 42%	0 94% 82%	text-git-deleted	File tree labels, PR closed icon
--git-modified	17 88% 40%	31 97% 72%	text-git-modified	File tree labels (M/R)
--git-merged	272 72% 47%	269 97% 85%	text-git-merged	PR merged icon
--git-added-bright	142 72% 29%	142 71% 45%	text-git-added-bright	+N stat text
--git-deleted-bright	0 74% 42%	0 91% 71%	text-git-deleted-bright	−N stat text
--surface-git-added	141 84% 93%	145 80% 10%	bg-surface-git-added	Stat badge background
--surface-git-deleted	0 93% 94%	0 75% 15%	bg-surface-git-deleted	Stat badge background

The bright variants use a more saturated dark-mode shade for stat
numbers (green-500/red-400 vs green-300/red-300), matching
the original styling. Light mode values are identical.

PR icon color changes

PR State	Before	After
Open	text-green-500	text-git-added
Closed	text-content-destructive	text-git-deleted
Merged	text-purple-500	text-git-merged
Draft	text-content-secondary	unchanged

Shared utility: diffColors.ts

Extracts duplicated changeColor() and changeLabel() functions from
DiffViewer.tsx and FilesChangedPanel.tsx into a single module.

Files changed

• index.css — new CSS variables (light + dark)
• tailwind.config.js — git.* and surface.git-* color mappings
• diffColors.ts — shared changeColor() / changeLabel()
• AgentsSidebar.tsx — PR icons + stat text use new tokens
• DiffStats.tsx — stat badges use new tokens
• DiffViewer.tsx — uses shared changeColor() / changeLabel()
• FilesChangedPanel.tsx — uses shared changeColor() / changeLabel()
• SidebarTabView.stories.tsx — stat badges use new tokens

[kylecarbs]

Lovely!