feat: add systemMessage support for SessionStart hook and tune defaults

Add systemMessage field to HookResult so SessionStart can display a
colored timeline directly to the user in the CLI. The handler now
parallel-fetches both markdown (for Claude context) and ANSI-colored
(for user display) timelines, appending a viewer URL link.

Also update default settings to hide verbose token columns (read/work
tokens, savings amount) and disable full observation expansion, keeping
the cleaner index-only view by default.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/cli/types.ts

@@ -16,6 +16,7 @@ export interface HookResult {
   continue?: boolean;
   suppressOutput?: boolean;
   hookSpecificOutput?: { hookEventName: string; additionalContext: string };
+  systemMessage?: string;
   exitCode?: number;
 }