fix: GitHub Copilot fallback — remove stale API version header and fix model name mapping

Two issues prevented the GitHub Models fallback from working:

1. The X-GitHub-Api-Version: 2022-11-28 header caused '400 invalid
   apiVersion' errors. The Copilot chat completions endpoint does not
   use this header — removed from both constructor and rebuildClient.

2. The anthropicToGitHubModel mapping was incomplete: it only knew
   three models and the generic date-stripping fallback produced wrong
   names (e.g. 'claude-sonnet-4-5' instead of 'claude-sonnet-4.5').
   GitHub Copilot uses dots for sub-versions, not hyphens.

   Updated with explicit mappings for all current models (sonnet 4,
   4.5; opus 4, 4.5, 4.6; haiku 4.5) and a smarter generic fallback
   that converts digit-hyphen-digit to digit.digit at the end.

3. createClientFromConfig now auto-maps Anthropic-style model names
   when the provider is 'github', so users can copy model names from
   their Anthropic config into fallback blocks without manual renaming.

src/daemon/clientFactory.test.ts

@@ -94,6 +94,23 @@ describe('createClientFromConfig', () => {
     });
     expect(client).toBeInstanceOf(BedrockClient);
   });
+
+  it('creates GitHubModelsClient for github provider', () => {
+    const client = createClientFromConfig({
+      provider: 'github',
+      model: 'claude-sonnet-4.5',
+    });
+    expect(client).toBeInstanceOf(GitHubModelsClient);
+  });
+
+  it('auto-maps Anthropic model names to GitHub equivalents for github provider', () => {
+    // User might copy-paste the Anthropic model name into a github fallback block
+    const client = createClientFromConfig({
+      provider: 'github',
+      model: 'claude-sonnet-4-5-20250929',
+    });
+    expect(client).toBeInstanceOf(GitHubModelsClient);
+  });
 });
 
 describe('anthropicToGitHubModel', () => {
@@ -101,15 +118,37 @@ describe('anthropicToGitHubModel', () => {
     expect(anthropicToGitHubModel('claude-sonnet-4-20250514')).toBe('claude-sonnet-4');
   });
 
+  it('maps claude-sonnet-4-5-20250929 to claude-sonnet-4.5', () => {
+    expect(anthropicToGitHubModel('claude-sonnet-4-5-20250929')).toBe('claude-sonnet-4.5');
+  });
+
   it('maps claude-opus-4-20250514 to claude-opus-4', () => {
     expect(anthropicToGitHubModel('claude-opus-4-20250514')).toBe('claude-opus-4');
   });
 
-  it('maps claude-3-5-haiku-20241022 to claude-haiku-4', () => {
-    expect(anthropicToGitHubModel('claude-3-5-haiku-20241022')).toBe('claude-haiku-4');
+  it('maps claude-opus-4-5-20250918 to claude-opus-4.5', () => {
+    expect(anthropicToGitHubModel('claude-opus-4-5-20250918')).toBe('claude-opus-4.5');
   });
 
-  it('strips date suffix for unknown versioned models', () => {
+  it('maps claude-opus-4-6-20250715 to claude-opus-4.6', () => {
+    expect(anthropicToGitHubModel('claude-opus-4-6-20250715')).toBe('claude-opus-4.6');
+  });
+
+  it('maps claude-3-5-haiku-20241022 to claude-haiku-4.5', () => {
+    expect(anthropicToGitHubModel('claude-3-5-haiku-20241022')).toBe('claude-haiku-4.5');
+  });
+
+  it('maps claude-haiku-4-5-20251001 to claude-haiku-4.5', () => {
+    expect(anthropicToGitHubModel('claude-haiku-4-5-20251001')).toBe('claude-haiku-4.5');
+  });
+
+  it('strips date suffix and converts trailing version number with dot for unknown models', () => {
+    // "claude-sonnet-5-7-20260101" → strip date → "claude-sonnet-5-7" → dot → "claude-sonnet-5.7"
+    expect(anthropicToGitHubModel('claude-sonnet-5-7-20260101')).toBe('claude-sonnet-5.7');
+  });
+
+  it('strips date suffix for models without sub-version', () => {
+    // "claude-sonnet-5-20260101" → strip date → "claude-sonnet-5" (no trailing -N to dot-convert)
     expect(anthropicToGitHubModel('claude-sonnet-5-20260101')).toBe('claude-sonnet-5');
   });