Explain: Fix passthrough of environment to Copilot CLI.

This bug caused errors of the form:

```
env: 'node': No such file or directory
```

To appear when Node was installed at a non-distro path (i.e. installed manually
or using NVM, rather than at `/usr/bin/node` by the distro's package manager).

This occurred because the `#!/usr/bin/env node` at the start of Copilot's
`index.js` couldn't guess a PATH to node.  With a distro-based install, env will
sometimes guess standard paths for you.

This was always a bug in my code but didn't manifest until we saw the right host
environment.

Author: mark-undoio

explain/copilot_cli_agent.py

@@ -105,13 +105,13 @@ async def ask(self, question: str, port: int, tools: list[str]) -> str:
             prompt = question
 
         allowed_tools = ["UDB_Server", "shell(grep)", "shell(find)", "shell(cat)", "shell(xargs)"]
-        env = {
+        env_changes = {
             "XDG_CONFIG_HOME": str(self._tempdir),
             "XDG_STATE_HOME": str(self._tempdir),
         }
         if runtime_dir := os.environ.get("XDG_RUNTIME_DIR"):
             # Pass through runtime state so it has auth access.
-            env["XDG_RUNTIME_DIR"] = runtime_dir
+            env_changes["XDG_RUNTIME_DIR"] = runtime_dir
 
         try:
             copilot = await asyncio.create_subprocess_exec(
@@ -127,7 +127,7 @@ async def ask(self, question: str, port: int, tools: list[str]) -> str:
                 "claude-sonnet-4.5",
                 "-p",
                 prompt,
-                env=env,
+                env=os.environ.copy().update(env_changes),
                 stdin=asyncio.subprocess.PIPE,
                 stdout=asyncio.subprocess.PIPE,
                 stderr=asyncio.subprocess.PIPE,