feat: sync PROVIDER_NAMES with config schema and update README docs

Extract MODEL_PROVIDERS const from config schema as single source of truth
for provider names. PROVIDER_NAMES in TUI commands now imports from schema
instead of maintaining a hardcoded list. Adds tests verifying sync.

Updates README TUI Commands section with /model hot-swap documentation,
supported providers, and runtime model switching examples.

src/frontends/tui/commands.test.ts

@@ -1,5 +1,6 @@
 import { describe, it, expect } from 'vitest';
-import { parseCommand, getHelpText } from './commands.js';
+import { parseCommand, getHelpText, getCommandCompletions, PROVIDER_NAMES } from './commands.js';
+import { MODEL_PROVIDERS } from '../../config/index.js';
 
 describe('parseCommand', () => {
   it('parses /quit command', () => {
@@ -130,3 +131,33 @@ describe('/pair command', () => {
     expect(parseCommand('/pair revoke telegram 12345')).toEqual({ type: 'pair', action: 'revoke', args: 'telegram 12345' });
   });
 });
+
+describe('PROVIDER_NAMES', () => {
+  it('matches all providers from the config schema', () => {
+    expect(PROVIDER_NAMES).toEqual(MODEL_PROVIDERS);
+  });
+
+  it('includes zhipuai, openrouter, and xai', () => {
+    expect(PROVIDER_NAMES).toContain('zhipuai');
+    expect(PROVIDER_NAMES).toContain('openrouter');
+    expect(PROVIDER_NAMES).toContain('xai');
+  });
+});
+
+describe('getCommandCompletions', () => {
+  it('suggests all providers when typing /model default with partial provider', () => {
+    // Typing any character after the tier triggers provider suggestions
+    const completions = getCommandCompletions('/model default a');
+    expect(completions).toContain('/model default anthropic');
+  });
+
+  it('filters provider suggestions by partial input', () => {
+    const completions = getCommandCompletions('/model default zh');
+    expect(completions).toEqual(['/model default zhipuai']);
+  });
+
+  it('completes xai provider', () => {
+    const completions = getCommandCompletions('/model fast x');
+    expect(completions).toEqual(['/model fast xai']);
+  });
+});