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]>

src/agent/dispatcher.rs

@@ -131,6 +131,17 @@ impl Agent {
             JobContext::with_user(&message.user_id, "chat", "Interactive chat session");
         job_ctx.http_interceptor = self.deps.http_interceptor.clone();
 
+        // Build system prompts once for this turn. Two variants: with tools
+        // (normal iterations) and without (force_text final iteration).
+        let initial_tool_defs = self.tools().tool_definitions().await;
+        let initial_tool_defs = if !active_skills.is_empty() {
+            crate::skills::attenuate_tools(&initial_tool_defs, &active_skills).tools
+        } else {
+            initial_tool_defs
+        };
+        let cached_prompt = reasoning.build_system_prompt_with_tools(&initial_tool_defs);
+        let cached_prompt_no_tools = reasoning.build_system_prompt_with_tools(&[]);
+
         let max_tool_iterations = self.config.max_tool_iterations;
         // Force a text-only response on the last iteration to guarantee termination
         // instead of hard-erroring. The penultimate iteration also gets a nudge
@@ -208,10 +219,16 @@ impl Agent {
             };
 
             // Call LLM with current context; force_text drops tools to guarantee a
-            // text response on the final iteration.
+            // text response on the final iteration. The pre-built system prompt
+            // avoids rebuilding the same ~1,500-token string each iteration.
             let mut context = ReasoningContext::new()
                 .with_messages(context_messages.clone())
                 .with_tools(tool_defs)
+                .with_system_prompt(if force_text {
+                    cached_prompt_no_tools.clone()
+                } else {
+                    cached_prompt.clone()
+                })
                 .with_metadata({
                     let mut m = std::collections::HashMap::new();
                     m.insert("thread_id".to_string(), thread_id.to_string());
@@ -248,7 +265,7 @@ impl Agent {
                     // Compact: keep system messages + last user message + current turn
                     context_messages = compact_messages_for_retry(&context_messages);
 
-                    // Rebuild context with compacted messages
+                    // Rebuild context with compacted messages, reusing cached prompt
                     let mut retry_context = ReasoningContext::new()
                         .with_messages(context_messages.clone())
                         .with_tools(if force_text {
@@ -258,6 +275,7 @@ impl Agent {
                         })
                         .with_metadata(context.metadata.clone());
                     retry_context.force_text = force_text;
+                    retry_context.system_prompt = context.system_prompt.clone();
 
                     reasoning
                         .respond_with_tools(&retry_context)