feat: ACP base provider, Copilot integration, eliminate hardcoded provider unions (#170)

* feat(server): add ACP base provider with Claude ACP integration

Implement a generic Agent Client Protocol (ACP) base provider that any
ACP-compatible agent can extend with minimal code. Includes a concrete
Claude ACP implementation via @agentclientprotocol/claude-agent-acp
with full parity to the existing Claude Code provider.

The base handles subprocess lifecycle, streaming translation, permission
bridging, terminal/fs callbacks, listModels, loadSession/resume, and
mode/model management. The concrete class only specifies the command,
modes, and availability check.

* fix: update lockfile signatures and Nix hash

* feat: add Copilot ACP provider, eliminate hardcoded provider unions, fix ACP streaming bugs

Add Copilot as an ACP provider (copilot --acp), with real modes and models
discovered from the ACP server. Fix two ACP base bugs: duplicate assistant
text (emit deltas not cumulative) and idle→running→stuck (fire-and-return
startTurn). Replace all hardcoded provider string unions with string/manifest-
derived values so adding a provider only requires: impl class, manifest entry,
registry factory, icon, and E2E config. Add provider docs and Copilot icon.

* fix: update lockfile signatures and Nix hash

* feat(app): add OpenCode provider icon

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

packages/cli/tests/15-provider.test.ts

@@ -54,7 +54,7 @@ let claudeModelsFromJson: ProviderModel[] = [];
 const ctx = await createE2ETestContext({ timeout: 120000 });
 
 async function runProviderModelsJson(
-  provider: "claude" | "codex" | "opencode",
+  provider: string,
 ): Promise<ProviderModel[]> {
   const transientNeedles = ["transport closed", "timed out", "timeout", "socket", "econn"];
 

packages/cli/tests/e2e/run-output-schema.test.ts

@@ -50,7 +50,7 @@ async function cleanup(): Promise<void> {
 }
 
 async function runProviderCase(input: {
-  provider: "claude" | "codex" | "opencode";
+  provider: string;
   mode: string;
   model: string;
 }): Promise<void> {