fix: avoid bash commands that may not be installed

Agent runs picked `file` to inspect template readability and hit exit 127
`/bin/sh: 1: file: not found`. The bash tool guidance only covered using
dedicated tools over cat/sed/grep, with no rule about missing commands.

Tell the model to inspect file existence/readability with ReadFile or basic
commands (`ls -l`, `stat`, `head`), and to switch away from a command that
exits 127 instead of retrying it.

Author: ruanzhengxin-zhuxing

src/iac_code/agent/system_prompt.py

@@ -187,6 +187,11 @@ def _build_tools_section() -> str:
         "  - Glob instead of find/ls\n"
         "  - Grep instead of grep/rg\n"
         "- Reserve Bash exclusively for system commands and terminal operations.\n"
+        "- To inspect whether a file exists or is readable, use ReadFile, or basic Bash commands "
+        "such as `ls -l`, `stat` or `head`. Do not rely on commands that may not be installed, "
+        "such as `file`.\n"
+        "- If a Bash command exits with 127 or `not found`, the command is missing from this "
+        "environment. Switch to a basic command or a dedicated tool instead of retrying it.\n"
         "- When calling multiple independent tools, make all calls in parallel.\n"
         "- Read files before modifying them.\n"
         "- Use EditFile for surgical edits to existing files.\n"

tests/agent/test_system_prompt.py

@@ -11,6 +11,7 @@
     SystemPromptBuilder,
     _build_cloud_config_section,
     _build_environment_section,
+    _build_tools_section,
     build_base_sections,
     build_system_prompt,
     split_by_dynamic_boundary,
@@ -437,3 +438,15 @@ def test_section_priorities_has_all_keys(self):
     def test_unknown_section_key_ignored(self):
         result = build_base_sections(["identity", "nonexistent_section"], cwd="/tmp")
         assert "Infrastructure as Code" in result
+
+
+class TestBuildToolsSection:
+    def test_file_inspection_prefers_basic_commands(self):
+        result = _build_tools_section()
+        assert "`ls -l`, `stat` or `head`" in result
+        assert "such as `file`" in result
+
+    def test_missing_command_exit_code_guidance(self):
+        result = _build_tools_section()
+        assert "exits with 127" in result
+