fix(learn): handle FileNotFoundError when CLI tool is not installed

When --model codex-cli is used but codex is not in PATH,
subprocess.run raises FileNotFoundError. Catch it and raise
a clear RuntimeError with guidance.

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

headroom/learn/analyzer.py

@@ -361,7 +361,7 @@ def _call_cli_llm(digest: str, model: str) -> dict:
 
     Raises:
         ValueError: If *model* is not a known CLI backend.
-        RuntimeError: If the CLI exits with a non-zero code or times out.
+        RuntimeError: If the CLI is not installed, exits non-zero, or times out.
     """
     cmd: list[str] | None = None
     for _name, model_name, cmd_parts in _CLI_BACKENDS:
@@ -381,6 +381,11 @@ def _call_cli_llm(digest: str, model: str) -> dict:
             text=True,
             timeout=_CLI_TIMEOUT,
         )
+    except FileNotFoundError:
+        raise RuntimeError(
+            f"`{cmd[0]}` not found in PATH. Install it or use a different backend "
+            "with --model <litellm-model-name>."
+        ) from None
     except subprocess.TimeoutExpired:
         raise RuntimeError(
             f"`{' '.join(cmd)}` did not respond within {_CLI_TIMEOUT}s. "

tests/test_learn/test_analyzer.py

@@ -547,6 +547,12 @@ class TestCallCliLlm:
         result = _call_cli_llm("test digest", "claude-cli")
         assert result == {"context_file_rules": [], "memory_file_rules": []}
 
+    @patch("headroom.learn.analyzer.subprocess.run")
+    def test_cli_not_installed_raises(self, mock_run: MagicMock):
+        mock_run.side_effect = FileNotFoundError("No such file or directory: 'codex'")
+        with pytest.raises(RuntimeError, match="not found in PATH"):
+            _call_cli_llm("test digest", "codex-cli")
+
     @patch("headroom.learn.analyzer.subprocess.run")
     def test_timeout_raises_runtime_error(self, mock_run: MagicMock):
         mock_run.side_effect = subprocess.TimeoutExpired(cmd=["claude", "-p"], timeout=120)