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, 0 insertions, 35 deletions

diff --git a/internal/llm/copilot_test.go b/internal/llm/copilot_test.go
deleted file mode 100644
index 8f15347..0000000
--- a/internal/llm/copilot_test.go
+++ /dev/null
@@ -1,35 +0,0 @@
-package llm
-
-import "testing"
-
-func TestBuildCopilotChatRequest_FieldsAndDefaults(t *testing.T) {
-	o := Options{
-		Model:       "gpt-x",
-		Temperature: 0,
-		MaxTokens:   123,
-		Stop:        []string{"X"},
-	}
-
-	msgs := []Message{{Role: "user", Content: "q"}}
-	req := buildCopilotChatRequest(o, msgs, f64p(0.5))
-
-	if req.Model != "gpt-x" {
-		t.Fatalf("model mismatch: %q", req.Model)
-	}
-
-	if req.Temperature == nil || *req.Temperature != 0.5 {
-		t.Fatalf("default temp not applied")
-	}
-
-	if req.MaxTokens == nil || *req.MaxTokens != 123 {
-		t.Fatalf("max_tokens not applied")
-	}
-
-	if len(req.Stop) != 1 || req.Stop[0] != "X" {
-		t.Fatalf("stop not applied")
-	}
-
-	if len(req.Messages) != 1 || req.Messages[0].Content != "q" {
-		t.Fatalf("messages not copied")
-	}
-}