Remove GitHub Copilot provider support

Remove all GitHub Copilot integration from the codebase to streamline
the supported provider set to OpenAI, OpenRouter, Anthropic, and Ollama.

Changes:
- Delete core Copilot implementation (copilot.go) and all related tests
- Remove Copilot configuration fields from App struct and Config
- Remove Copilot from provider factory and API key handling
- Update all test files to replace Copilot references with other providers
- Remove Copilot documentation from README, configuration guide, and examples
- Remove Copilot section from config.toml.example

All tests pass successfully after removal.

Co-authored-by: Cursor <cursoragent@cursor.com>

1 files changed, 2 insertions, 6 deletions

diff --git a/internal/llm/provider_test.go b/internal/llm/provider_test.go
index 46c7ea8..8ccba6e 100644
--- a/internal/llm/provider_test.go
+++ b/internal/llm/provider_test.go
@@ -6,15 +6,11 @@ import (
 
 func TestNewFromConfig_DefaultsAndErrors(t *testing.T) {
 	// Unknown provider
-	if _, err := NewFromConfig(Config{Provider: "bogus"}, "", "", "", ""); err == nil {
+	if _, err := NewFromConfig(Config{Provider: "bogus"}, "", "", ""); err == nil {
 		t.Fatalf("expected error for unknown provider")
 	}
 	// OpenAI missing key
-	if _, err := NewFromConfig(Config{Provider: "openai", OpenAIModel: "g"}, "", "", "", ""); err == nil {
-		t.Fatalf("expected key error")
-	}
-	// Copilot missing key
-	if _, err := NewFromConfig(Config{Provider: "copilot", CopilotModel: "m"}, "", "", "", ""); err == nil {
+	if _, err := NewFromConfig(Config{Provider: "openai", OpenAIModel: "g"}, "", "", ""); err == nil {
 		t.Fatalf("expected key error")
 	}
 }