keymap: Colemak UML layer with arrows, BT combos, fix BLE/settings config

- UML layer: arrows on T(↑) C(←) D(↓) V(→), ALT on Enter
- BT profile switching via combos: CTRL+Q/W/F/P → profiles 1-4
- Explicit CONFIG_ZMK_BLE, CONFIG_FLASH, CONFIG_NVS, CONFIG_SETTINGS_NVS
- Board name: seeeduino_xiao_ble → xiao_ble
- Add .gitignore, AGENTS.md (build docs), BINDINGS.md (keymap ref)

.gitignore

@@ -0,0 +1,18 @@
+# Build artifacts
+build/
+*.uf2
+
+# West-managed submodules
+modules/
+optional/
+zephyr/
+zmk/
+
+# West state
+.west/
+
+# macOS
+.DS_Store
+
+# Reasonix
+.reasonix/