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>

package-lock.json

@@ -48,6 +48,15 @@
         }
       }
     },
+    "node_modules/@agentclientprotocol/sdk": {
+      "version": "0.17.1",
+      "resolved": "https://registry.npmjs.org/@agentclientprotocol/sdk/-/sdk-0.17.1.tgz",
+      "integrity": "sha512-yjyIn8POL18IOXioLySYiL0G44kZ/IZctAls7vS3AC3X+qLhFXbWmzABSZehwRnWFShMXT+ODa/HJG1+mGXZ1A==",
+      "license": "Apache-2.0",
+      "peerDependencies": {
+        "zod": "^3.25.0 || ^4.0.0"
+      }
+    },
     "node_modules/@ai-sdk/gateway": {
       "version": "2.0.1",
       "resolved": "https://registry.npmjs.org/@ai-sdk/gateway/-/gateway-2.0.1.tgz",
@@ -35416,6 +35425,7 @@
       "name": "@getpaseo/server",
       "version": "0.1.43-rc.1",
       "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",