fix(core): gate hybrid FTS relaxation with the service's eligibility rules

Addresses Codex review on #994: the hybrid FTS branch opted into
OR-relaxation for every query shape, but SearchService's relaxed FTS
path deliberately rejects short queries and numeric identifiers because
OR-relaxing them over-broadens — and in hybrid the relaxed FTS-only rows
normalize to 1.0 and can outrank the vector result the user wanted
(e.g. "SPEC 16", "root note 1", "New Feature").

relaxed_query_words now enforces the same eligibility as
SearchService._is_relaxed_fts_fallback_eligible: tokenize on
[A-Za-z0-9]+ and return None when there are fewer than three tokens or
any token is a pure digit (in addition to the existing quoted/boolean
guards). Both the SQLite and Postgres relaxed retries route through this
helper, so the hybrid path now relaxes exactly the query shapes the
service does.

Tests updated for the tightened eligibility (short + numeric queries no
longer relax); SQLite + Postgres relaxation suites green, full
repository/search-service/link-resolver suite green (442 passed), ty
clean.

Signed-off-by: Drew Cain <groksrc@gmail.com>

src/basic_memory/repository/search_repository_base.py

@@ -53,14 +53,28 @@ RELAXATION_STOPWORDS = frozenset(
 def relaxed_query_words(search_text: Optional[str]) -> Optional[list[str]]:
     """Content-bearing words for OR-relaxing a strict full-text query.
 
-    Returns None when relaxation must not apply: empty input, quoted phrases,
-    or explicit boolean queries (user intent is not second-guessed).
+    Returns None when relaxation must not apply. These eligibility rules match
+    SearchService._is_relaxed_fts_fallback_eligible so the hybrid FTS branch
+    relaxes exactly the same query shapes as the service-level FTS path:
+
+    - empty / quoted / explicit-boolean queries (user intent is not
+      second-guessed);
+    - fewer than three alphanumeric tokens (short queries like "New Feature"
+      over-broaden under OR — and in hybrid the relaxed FTS-only rows normalize
+      to 1.0 and can outrank the vector result the user wanted);
+    - any pure-digit token ("root note 1", "SPEC 16") — identifier-like queries
+      over-broaden and create false positives under OR.
     """
     if not search_text:
         return None
     stripped = search_text.strip()
     if '"' in stripped or any(op in f" {stripped} " for op in (" AND ", " OR ", " NOT ")):
         return None
+    # Eligibility checks run on raw alphanumeric tokens (parity with the
+    # service), before stopword filtering.
+    tokens = re.findall(r"[A-Za-z0-9]+", stripped.lower())
+    if len(tokens) < 3 or any(token.isdigit() for token in tokens):
+        return None
     words = [word.strip("?!.,;:") for word in stripped.split()]
     words = [
         word