feat(installer): add GitHub Copilot support with priority-based fallback

Add GitHub Copilot as a fallback provider option in the installer. Native providers (Claude/ChatGPT/Gemini) have priority over Copilot.

- Add hasCopilot field to InstallConfig and DetectedConfig types
- Add --copilot flag support for both TUI and non-TUI modes
- Add Copilot prompt in interactive installer (after Gemini)
- Update model selection logic to use Copilot fallback when native providers unavailable
- Update configuration summary to display Copilot status

Addresses #762

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/cli/types.ts

@@ -6,6 +6,7 @@ export interface InstallArgs {
   claude?: ClaudeSubscription
   chatgpt?: BooleanArg
   gemini?: BooleanArg
+  copilot?: BooleanArg
   skipAuth?: boolean
 }
 
@@ -14,6 +15,7 @@ export interface InstallConfig {
   isMax20: boolean
   hasChatGPT: boolean
   hasGemini: boolean
+  hasCopilot: boolean
 }
 
 export interface ConfigMergeResult {
@@ -28,4 +30,5 @@ export interface DetectedConfig {
   isMax20: boolean
   hasChatGPT: boolean
   hasGemini: boolean
+  hasCopilot: boolean
 }