fix: prevent write_interim_draft context overflow (465K → ≤300K chars)

Two bugs caused all 5 interim blocks to fail with "Claude CLI failed
(exit 1): unknown error":

1. source_context was embedded BOTH inside the prompt template (via
   {source_context}) AND prepended again in write_block — doubling every
   block's context size (232K chars × 2 = 465K chars).

2. _build_source_context loaded all 9 case documents for every block
   regardless of relevance.

Fixes:
- Remove the duplicate source_context prepend in write_block; the
  template already contains it via {source_context}
- Add per-block document filtering (_BLOCK_DOC_TYPES): block-he/zayin →
  empty, block-chet → protocol only, block-tet → appraisals only
- Add 400K char guard before calling claude -p with a descriptive error
  (vs opaque "exit 1: unknown error")
- Add prompt-size warning and size info in claude_session error messages

Result: block-he 0 chars, block-zayin 0 chars, block-vav ~172K,
block-chet ~45K, block-tet ~300K (all under 400K limit)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

mcp-server/src/legal_mcp/services/claude_session.py

@@ -72,6 +72,9 @@ async def query(
     """
     full_prompt = f"{system}\n\n{prompt}" if system else prompt
 
+    if len(full_prompt) > 150_000:
+        logger.warning("Large prompt: %d chars — may hit context limits", len(full_prompt))
+
     cmd = [
         "claude", "-p",
         "--output-format", "json",
@@ -110,7 +113,8 @@ async def query(
 
     if proc.returncode != 0:
         stderr = stderr_b.decode("utf-8", errors="replace").strip()[:500] or "unknown error"
-        raise RuntimeError(f"Claude CLI failed (exit {proc.returncode}): {stderr}")
+        size_info = f"; prompt_len={len(full_prompt):,} chars" if len(full_prompt) > 100_000 else ""
+        raise RuntimeError(f"Claude CLI failed (exit {proc.returncode}): {stderr}{size_info}")
 
     stdout = stdout_b.decode("utf-8", errors="replace").strip()
     if not stdout: