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/server/package.json

@@ -60,6 +60,7 @@
     "test:e2e:ui": "vitest --ui e2e.test.ts"
   },
   "dependencies": {
+    "@agentclientprotocol/sdk": "^0.17.1",
     "@ai-sdk/openai": "2.0.52",
     "@anthropic-ai/claude-agent-sdk": "^0.2.11",
     "@deepgram/sdk": "^3.4.0",