feat: add WSL environment hint to system prompt (#8285)

When running inside WSL (Windows Subsystem for Linux), inject a hint into
the system prompt explaining that the Windows host filesystem is mounted
at /mnt/c/, /mnt/d/, etc. This lets the agent naturally translate Windows
paths (Desktop, Documents) to their /mnt/ equivalents without the user
needing to configure anything.

Uses the existing is_wsl() detection from hermes_constants (cached,
checks /proc/version for 'microsoft'). Adds build_environment_hints()
in prompt_builder.py — extensible for Termux, Docker, etc. later.

Closes the UX gap where WSL users had to manually explain path
translation to the agent every session.

tests/agent/test_prompt_builder.py

@@ -18,6 +18,7 @@ from agent.prompt_builder import (
     build_skills_system_prompt,
     build_nous_subscription_prompt,
     build_context_files_prompt,
+    build_environment_hints,
     CONTEXT_FILE_MAX_CHARS,
     DEFAULT_AGENT_IDENTITY,
     TOOL_USE_ENFORCEMENT_GUIDANCE,
@@ -26,6 +27,7 @@ from agent.prompt_builder import (
     MEMORY_GUIDANCE,
     SESSION_SEARCH_GUIDANCE,
     PLATFORM_HINTS,
+    WSL_ENVIRONMENT_HINT,
 )
 from hermes_cli.nous_subscription import NousFeatureState, NousSubscriptionFeatures
 
@@ -770,6 +772,29 @@ class TestPromptBuilderConstants:
         assert "cli" in PLATFORM_HINTS
 
 
+# =========================================================================
+# Environment hints
+# =========================================================================
+
+class TestEnvironmentHints:
+    def test_wsl_hint_constant_mentions_mnt(self):
+        assert "/mnt/c/" in WSL_ENVIRONMENT_HINT
+        assert "WSL" in WSL_ENVIRONMENT_HINT
+
+    def test_build_environment_hints_on_wsl(self, monkeypatch):
+        import agent.prompt_builder as _pb
+        monkeypatch.setattr(_pb, "is_wsl", lambda: True)
+        result = _pb.build_environment_hints()
+        assert "/mnt/" in result
+        assert "WSL" in result
+
+    def test_build_environment_hints_not_wsl(self, monkeypatch):
+        import agent.prompt_builder as _pb
+        monkeypatch.setattr(_pb, "is_wsl", lambda: False)
+        result = _pb.build_environment_hints()
+        assert result == ""
+
+
 # =========================================================================
 # Conditional skill activation
 # =========================================================================