fix: clear CLAUDECODE env to prevent nested-session detection

When claude-agent-acp is spawned from within a Claude Code session
(e.g. via acpx or other orchestrators), the parent process sets
CLAUDECODE=1. This env var is inherited by the subprocess, causing
Claude Code to detect a "nested session" and exit immediately.
The result is a generic -32603 "Internal error" with details
"Query closed before response received" on every session/new call.

Fix: clear CLAUDECODE in the subprocess environment so the spawned
Claude Code instance starts normally.

Also improve error handling for session/new: the v0.20.0 fix for
"Query closed before response received" only covered the loadSession
path (resume). The same error on newSession was left unhandled,
producing an opaque "Internal error". Now it returns a meaningful
message suggesting to check Claude Code installation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sunnyyi-mac

src/acp-agent.ts

@@ -1227,6 +1227,7 @@ export class ClaudeAcpAgent implements Agent {
       ...userProvidedOptions,
       env: {
         ...process.env,
+        CLAUDECODE: "", // Prevent nested-session detection when spawned from within Claude Code
         ...userProvidedOptions?.env,
         ...createEnvForGateway(this.gatewayAuthMeta),
       },
@@ -1303,11 +1304,19 @@ export class ClaudeAcpAgent implements Agent {
       initializationResult = await q.initializationResult();
     } catch (error) {
       if (
-        creationOpts.resume &&
         error instanceof Error &&
         error.message === "Query closed before response received"
       ) {
-        throw RequestError.resourceNotFound(sessionId);
+        if (creationOpts.resume) {
+          throw RequestError.resourceNotFound(sessionId);
+        }
+        // For new sessions, provide a meaningful error instead of generic "Internal error"
+        throw RequestError.internalError(
+          undefined,
+          "Claude Code process exited before initialization completed. " +
+            "Ensure Claude Code is installed and accessible. " +
+            "You can set CLAUDE_CODE_EXECUTABLE to the path of the Claude CLI.",
+        );
       }
       throw error;
     }