feat: reduce term index noise — skip tool messages, filter numerics and JSON keys

Tool-role messages (44% of corpus) produce junk terms from structured
JSON output: field names like 'output', 'exit_code', 'error', 'status'
and numeric values like '0', '1', '42'. These have zero search value and
account for ~77% of index rows.

Changes:
- stop_words.py: add JSON key stop words (output, exit_code, error, null,
  true, false, status, content, message, cleared, success) and
  is_noise_term() that also filters pure numeric tokens
- term_index.py: switch from is_stop_word to is_noise_term
- hermes_state.py: skip tool-role messages at insert and reindex paths
- tests: 6 new TestNoiseReduction tests

Impact on live DB: 1.19M -> 278K rows (77% reduction), 1.5s reindex.

hermes_state.py

@@ -1076,7 +1076,9 @@ class SessionDB:
             # Insert terms into inverted index (delayed import avoids
             # circular dependency — hermes_state is imported by nearly
             # everything at startup, term_index must not be top-level)
-            if content:
+            # Skip tool-role messages: their structured JSON output produces
+            # noise terms (field names, numeric values) with no search value.
+            if content and role != "tool":
                 try:
                     from term_index import extract_terms
                     terms = extract_terms(content)
@@ -1605,7 +1607,7 @@ class SessionDB:
             # Read batch outside write lock
             with self._lock:
                 cursor = self._conn.execute(
-                    "SELECT id, session_id, content FROM messages ORDER BY id LIMIT ? OFFSET ?",
+                    "SELECT id, session_id, role, content FROM messages ORDER BY id LIMIT ? OFFSET ?",
                     (batch_size, offset),
                 )
                 rows = cursor.fetchall()
@@ -1613,11 +1615,14 @@ class SessionDB:
             if not rows:
                 break
 
-            # Extract terms for the batch
+            # Extract terms for the batch, skipping tool-role messages
             entries = []
             for row in rows:
                 msg_id = row["id"]
                 session_id = row["session_id"]
+                # Skip tool messages — structured JSON output produces noise terms
+                if row["role"] == "tool":
+                    continue
                 content = row["content"] or ""
                 terms = extract_terms(content)
                 for term in terms: