feat(run_agent): inject workspace guidance into system prompt

Wires build_workspace_guidance() into AIAgent._build_system_prompt()
alongside the existing memory/session_search/skills hooks. Guidance is
assembled dynamically from self.valid_tool_names and appended to the
tool_guidance section.

Closes the 0/6 workspace_search discoverability gap measured in the
2026-04-20 A/B dogfood.

run_agent.py

@@ -83,6 +83,7 @@ from agent.prompt_builder import (
     DEFAULT_AGENT_IDENTITY, PLATFORM_HINTS,
     MEMORY_GUIDANCE, SESSION_SEARCH_GUIDANCE, SKILLS_GUIDANCE,
     build_nous_subscription_prompt,
+    build_workspace_guidance,
 )
 from agent.model_metadata import (
     fetch_model_metadata,
@@ -3733,6 +3734,9 @@ class AIAgent:
             tool_guidance.append(SESSION_SEARCH_GUIDANCE)
         if "skill_manage" in self.valid_tool_names:
             tool_guidance.append(SKILLS_GUIDANCE)
+        workspace_block = build_workspace_guidance(set(self.valid_tool_names))
+        if workspace_block:
+            tool_guidance.append(workspace_block)
         if tool_guidance:
             prompt_parts.append(" ".join(tool_guidance))
 

tests/agent/test_prompt_builder.py

@@ -1122,3 +1122,35 @@ class TestWorkspaceGuidance:
         out = build_workspace_guidance({"workspace_search"})
         assert isinstance(out, str)
         assert len(out.strip()) > 0
+
+    def test_wiring_into_system_prompt(self):
+        """End-to-end: the assembler output reaches the system prompt when
+        workspace_search is in valid_tool_names.
+
+        Uses the same tool_guidance collection pattern as _build_system_prompt
+        in run_agent.py so we verify the contract without booting AIAgent.
+        """
+        from agent.prompt_builder import (
+            MEMORY_GUIDANCE,
+            build_workspace_guidance,
+        )
+        valid_tool_names = {"memory", "workspace_search", "workspace_retrieve"}
+        tool_guidance = []
+        if "memory" in valid_tool_names:
+            tool_guidance.append(MEMORY_GUIDANCE)
+        ws = build_workspace_guidance(valid_tool_names)
+        if ws:
+            tool_guidance.append(ws)
+        combined = " ".join(tool_guidance)
+        assert WORKSPACE_SEARCH_GUIDANCE_CORE in combined
+        assert WORKSPACE_RETRIEVE_GUIDANCE in combined
+        assert MEMORY_GUIDANCE in combined
+
+    def test_wiring_skips_when_workspace_unavailable(self):
+        valid_tool_names = {"memory"}
+        tool_guidance = []
+        ws = build_workspace_guidance(valid_tool_names)
+        if ws:
+            tool_guidance.append(ws)
+        combined = " ".join(tool_guidance)
+        assert WORKSPACE_SEARCH_GUIDANCE_CORE not in combined