Add Grok Build agent config adapted from Claude Code setup

Wire shared AGENTS.md and Claude settings through home-manager, plus a
native Grok config (always-approve, rose-pine) and a `gb` yolo alias.

home/.grok/config.toml

@@ -0,0 +1,28 @@
+# Grok Build user config - managed via home-manager (edit here, then rebuild if needed).
+# Adapted from Claude Code setup in kunchenguid/dotfiles + local preferences.
+#
+# Mapping notes:
+#   Claude --dangerously-skip-permissions  ->  permission_mode = "always-approve" / --yolo
+#   Claude theme dark-ansi + rose-pine stack ->  theme = "rosepine"
+#   Claude statusLine (model | ctx %)      ->  no direct equivalent; TUI shows this natively
+#   Shared AGENTS.md                       ->  ~/.grok/AGENTS.md (via home.nix)
+
+[marketplace]
+official_marketplace_auto_installed = true
+
+[[marketplace.sources]]
+name = "xAI Official"
+git = "https://github.com/xai-org/plugin-marketplace.git"
+
+[models]
+default = "grok-build"
+
+[ui]
+theme = "rosepine"
+max_thoughts_width = 120
+fork_secondary_model = "grok-build"
+# High-agency default, same idea as `claude --dangerously-skip-permissions` / `codex --full-auto`.
+# Toggle per session with Ctrl+O or /always-approve. Shell alias `gb` also launches with --yolo.
+permission_mode = "always-approve"
+yolo = false
+compact_mode = false