feat(tui): detect terminal color scheme and apply light-optimised palette

Squash of the linearized fix/tui-color-scheme-v2 merge and the follow-up
catalog regeneration. Renders error cause chains at every diagnostic seam
(origin/fetch-failed-diagnostics) and adds color scheme detection with a
light-terminal palette.

packages/context/workspace-context/src/render.ts

@@ -163,6 +163,12 @@ function buildInstructionText(
 ): string {
   const marker = markerText(maxBytes, omitted, truncated)
   const body = [marker, style.intro, ...files.map(file => style.section(file))].filter(block => block.length > 0)
+  // Caller-owned framing: the plugin bakes the complete `<system-reminder>`
+  // frame into the message content. The session surface projects context
+  // verbatim and does not wrap it, so any framing must live here in the
+  // producer's content (the pattern a future `meta`-driven renderer would
+  // generalize — see the deferred note in
+  // ../../../../.agents/notes/implemented/simplification/2026-07-20-unwrap-injected-content-envelopes.md).
   return [SYSTEM_REMINDER_OPEN, body.join('\n\n'), SYSTEM_REMINDER_CLOSE].join('\n')
 }