perf: build system prompt once per turn, skip tools on force-text (#583)

* perf: build system prompt once per turn, skip tools on force-text, fix nudge role (#565)

Three fixes to agentic loop prompt handling:

1. Build system prompt once per turn instead of every tool iteration.
   `build_system_prompt_with_tools` is now pub; callers pass the result
   via `ReasoningContext::system_prompt` to avoid rebuilding ~1,500 tokens
   per iteration.

2. Skip `## Available Tools` section when `force_text = true`. The
   dispatcher passes a no-tools prompt variant on the final iteration,
   saving ~460 tokens and removing misleading instructions.

3. Change nudge message from `Role::System` to `Role::User`. A second
   system message mid-conversation is unsupported by most providers.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

* fix: revert nudge role change to keep ChatMessage::system

Copilot review correctly identified that using Role::User for the nudge
breaks compact_messages_for_retry, which uses rposition for Role::User
to find the last real user message. Role::Assistant would cause
back-to-back assistant messages. Since no production issues were reported
with the original system role, revert to ChatMessage::system.

[skip-regression-check]

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

* fix: address PR review — omit tool guidance when tools empty, rename shadowed var

- Conditionalize "Call tools…" guidelines and "## Tool Call Style" section
  in the system prompt so they are only included when tools are non-empty.
  Previously the force-text (no-tools) prompt still contained misleading
  tool-calling instructions. (Copilot review comment)

- Rename `system_prompt` → `cached_prompt` in dispatcher to avoid shadowing
  the earlier workspace identity `system_prompt` variable. (Copilot review)

- Add regression tests: `test_system_prompt_with_tools_contains_tool_guidance`
  and extended assertions in `test_system_prompt_without_tools_omits_tools_section`.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <[email protected]>
Co-authored-by: [email protected] <[email protected]>

tests/support_unit_tests.rs

@@ -96,46 +96,42 @@ mod cleanup_tests {
 
     #[test]
     fn cleanup_guard_removes_file() {
-        let tmp = tempfile::tempdir().unwrap();
-        let path = tmp.path().join("cleanup_guard_test.txt");
-        std::fs::write(&path, "test").unwrap();
-        let path_str = path.to_str().unwrap().to_string();
+        let path = "/tmp/ironclaw_cleanup_guard_test.txt";
+        std::fs::write(path, "test").unwrap();
         {
-            let _guard = CleanupGuard::new().file(path_str);
-            assert!(path.exists());
+            let _guard = CleanupGuard::new().file(path);
+            assert!(std::path::Path::new(path).exists());
         }
-        assert!(!path.exists());
+        assert!(!std::path::Path::new(path).exists());
     }
 
     #[test]
     fn cleanup_guard_removes_dir() {
-        let tmp = tempfile::tempdir().unwrap();
-        let dir = tmp.path().join("cleanup_guard_test_dir");
-        std::fs::create_dir_all(&dir).unwrap();
-        std::fs::write(dir.join("file.txt"), "test").unwrap();
-        let dir_str = dir.to_str().unwrap().to_string();
+        let dir = "/tmp/ironclaw_cleanup_guard_test_dir";
+        std::fs::create_dir_all(dir).unwrap();
+        std::fs::write(format!("{dir}/file.txt"), "test").unwrap();
         {
-            let _guard = CleanupGuard::new().dir(dir_str);
-            assert!(dir.exists());
+            let _guard = CleanupGuard::new().dir(dir);
+            assert!(std::path::Path::new(dir).exists());
         }
-        assert!(!dir.exists());
+        assert!(!std::path::Path::new(dir).exists());
     }
 
     #[test]
     fn cleanup_guard_file_does_not_remove_dir() {
-        let tmp = tempfile::tempdir().unwrap();
-        let dir = tmp.path().join("cleanup_guard_file_not_dir");
-        std::fs::create_dir_all(&dir).unwrap();
-        let dir_str = dir.to_str().unwrap().to_string();
+        let dir = "/tmp/ironclaw_cleanup_guard_file_not_dir";
+        std::fs::create_dir_all(dir).unwrap();
         {
             // Registering a directory path as .file() should not remove it
             // (remove_file fails on directories).
-            let _guard = CleanupGuard::new().file(dir_str);
+            let _guard = CleanupGuard::new().file(dir);
         }
         assert!(
-            dir.exists(),
+            std::path::Path::new(dir).exists(),
             "dir should still exist when registered as file"
         );
+        // Clean up manually.
+        let _ = std::fs::remove_dir_all(dir);
     }
 }