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/.claude/settings.json

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+{
+  "theme": "dark-ansi",
+  "statusLine": {
+    "type": "command",
+    "command": "input=$(cat); model=$(echo \"$input\" | jq -r '.model.display_name'); used=$(echo \"$input\" | jq -r '.context_window.used_percentage // empty'); if [ -n \"$used\" ]; then printf \"%s | ctx: %.0f%% used\" \"$model\" \"$used\"; else printf \"%s\" \"$model\"; fi"
+  }
+}

home/.grok/config.toml

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+# 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

home/AGENTS.md

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+# global agent instructions
+
+- Never use the em dash "—". Use plain dash "-" instead
+- When writing commit messages, NEVER auto-add your agent name as co-author
+- Never manually modify CHANGELOG.md files or any files that are marked as auto-generated
+- When making technical decisions, do not give much weight to development cost.
+  Instead, prefer quality, simplicity, robustness, scalability, and long term maintainability.
+- When doing bug fixes, always start with reproducing the bug in an E2E setting as closely aligned with how an end user would experience it as possible.
+  This makes sure you find the real problem so your fix will actually solve it.
+- When end-to-end testing a product, be picky about the UI you see and be obsessed with pixel perfection.
+  If something clearly looks off, even if it is not directly related to what you are doing, try to get it fixed along the way.
+- Apply that same high standard to engineering excellence: lint, test failures, and test flakiness.
+  If you see one, even if it is not caused by what you are working on right now, still get it fixed.
+- Before using "dynamic workflows", "ultra code" or any harness feature that immediately spawns a large swarm of subagents, always explain the tradeoffs and ask the user for explicit approval.