fix: write system prompts to file to avoid Windows command line limit

Changes:
- Write system prompts to CLAUDE.md file instead of passing inline
- Use setting_sources=["project"] to load prompts from file
- Affects spec_chat_session.py and assistant_chat_session.py

Why:
- Windows has ~8191 character command line limit
- System prompts (e.g., create-spec.md at ~19KB) exceeded this limit
- The SDK serializes system_prompt as a CLI argument
- Writing to file and using setting_sources bypasses the limit

This completes the fix for GitHub issue #33 (Windows "Command Line Too Long").
The previous commit removed **os.environ from MCP configs; this commit
addresses the larger system prompt issue.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

server/services/spec_chat_session.py

@@ -154,6 +154,13 @@ class SpecChatSession:
         project_path = str(self.project_dir.resolve())
         system_prompt = skill_content.replace("$ARGUMENTS", project_path)
 
+        # Write system prompt to CLAUDE.md file to avoid Windows command line length limit
+        # The SDK will read this via setting_sources=["project"]
+        claude_md_path = self.project_dir / "CLAUDE.md"
+        with open(claude_md_path, "w", encoding="utf-8") as f:
+            f.write(system_prompt)
+        logger.info(f"Wrote system prompt to {claude_md_path}")
+
         # Create Claude SDK client with limited tools for spec creation
         # Use Opus for best quality spec generation
         # Use system Claude CLI to avoid bundled Bun runtime crash (exit code 3) on Windows
@@ -167,7 +174,9 @@ class SpecChatSession:
                 options=ClaudeAgentOptions(
                     model="claude-opus-4-5-20251101",
                     cli_path=system_cli,
-                    system_prompt=system_prompt,
+                    # System prompt loaded from CLAUDE.md via setting_sources
+                    # This avoids Windows command line length limit (~8191 chars)
+                    setting_sources=["project"],
                     allowed_tools=[
                         "Read",
                         "Write",