allozaur/feat/chat slash commands (#26716)

* base : slash-command/misc foundation - model icon and focus-selector constants

* feat : slash-command picker and command parsing helpers

* refactor : wire command and @-mention pickers into the chat form

* ui : improve model selector keyboard navigation and load/dismiss

* feat: Unify markdown/raw-text rendering under one setting with migration

* fix: Misc fixes - tool-call subtitle, assistant wrap, progress guards

* feat: Clamp and style numeric settings inputs from registry bounds

tools/ui/tests/client/components/ChatFormPickersHarness.svelte

@@ -0,0 +1,51 @@
+<script lang="ts">
+	import {
+		useChatFormPickers,
+		type UseChatFormPickersReturn
+	} from '$lib/hooks/use-chat-form-pickers.svelte';
+
+	let value = $state('');
+	let caretOffset = $state(0);
+	const calls: string[] = [];
+
+	const pickers = useChatFormPickers({
+		getValue: () => value,
+		setValue: (v) => {
+			value = v;
+			calls.push(`setValue:${v}`);
+		},
+		getCaretOffset: () => caretOffset,
+		setCaretOffset: (o) => {
+			caretOffset = o;
+		},
+		focusInput: () => {},
+		getShowModelSelector: () => true,
+		hasPrompts: () => true,
+		hasBuiltinTools: () => true,
+		getCwd: () => null,
+		getServerHome: () => null,
+		openModelSelector: () => {
+			calls.push('openModelSelector');
+		},
+		getPickersRef: () => undefined
+	});
+
+	// Simulate the user typing: update the buffer and run the input flow.
+	export function type(text: string) {
+		value = text;
+		caretOffset = text.length;
+		pickers.handleInput();
+	}
+
+	export function getValue() {
+		return value;
+	}
+
+	export function getCalls() {
+		return calls;
+	}
+
+	export function getPickers(): UseChatFormPickersReturn {
+		return pickers;
+	}
+</script>