Fix Claude Code Home Manager config

Generate Claude settings from Nix so managed commands use the correct home directory, add declarative keybindings for Shift+Enter newline support, and replace the broken inline docs hook with a portable script.

Also stop exporting CLAUDE_CONFIG_DIR so Claude no longer splits managed config between ~/.claude and ~/.config/claude, and fix the status line script shebang for NixOS.

Co-authored-by: Codex <noreply@openai.com>

config/claude/keybindings.json

@@ -0,0 +1,12 @@
+{
+  "$schema": "https://www.schemastore.org/claude-code-keybindings.json",
+  "$docs": "https://code.claude.com/docs/en/keybindings",
+  "bindings": [
+    {
+      "context": "Chat",
+      "bindings": {
+        "shift+enter": "chat:newline"
+      }
+    }
+  ]
+}