Fix brand color tokens keeping light values in dark mode

[calvintvu]

Context

In dark mode, the cards on the subagent rail still rendered in the light palette. The cause is in src/index.css. The 21 coSlash brand tokens were literal hex values inside a plain @theme block. A literal in @theme has nothing for .dark to override, so every brand token kept its light value on a dark page. The subagent card was the most visible case, because it is the largest painted surface and it repeats once per subagent.

Changes

• All 21 brand tokens now follow the theme. The @theme block became @theme inline aliases that point at :root variables, plus a .dark override set. This is the same indirection the shadcn neutrals already use in this file. No component reads these through anything but a utility class, so no consumer changed.
• The subagent card lifts off the page instead of mirroring light mode. In light mode the card sits 1.05 below a white page. Below #0a0a0a is mud, so on dark it sits 1.06 above the page at #0c1315.
• The card tint stays a whisper on purpose. DetailedSubagentRow paints the subagent-bg badge directly onto the card, and a saturated card swallows that badge. The rail carries the subagent identity instead, at #2a94a8.
• The sheet scrim gets a dark value. It was a hardcoded light navy rgba(23, 39, 66, 0.28), which does nothing over a near-black page. On dark it is now rgba(0, 0, 0, 0.6).
• The subagent digest row in SessionInspector no longer uses border-subagent-rail as a full box border. That token becomes strong teal on dark, which would ring the row in loud teal. The row now uses the default card border, which matches SubagentCard.
• Light mode is unchanged. Every :root value is the same hex as before.

Test

• npm run lint — passed. Two warnings remain in SessionSortDropdownMenu.tsx, both present before this branch.
• npx tsc --noEmit — passed
• npm test — passed, 10 tests
• npx vite build — passed. In the built CSS, .bg-subagent-card now emits background-color:var(--subagent-card) instead of a baked hex, :root keeps #f2fbfc, and .dark carries #0c1315.

Screenshots