fix(seahorse): correct bm25 rank semantics in comments (#2360)

SQLite FTS5 bm25() returns negative values where numerically smaller
(more negative) indicates a better match. The official docs state:

  "The better the match, the numerically smaller the value returned."

Two comments incorrectly stated "closer to 0 = better match" and
"lower = better match". Updated all rank descriptions to use the
unambiguous "more negative = higher relevance" phrasing.

This matters because these comments are used as tool prompt hints
for LLM agents, and incorrect semantics could lead to wrong ranking
decisions.

pkg/seahorse/short_retrieval.go

@@ -68,8 +68,8 @@ type GrepSummaryResult struct {
 	Depth          int         `json:"depth"`
 	Kind           SummaryKind `json:"kind"`
 	ConversationID int64       `json:"conversationId"`
-	// Rank is the bm25 relevance score (negative value, closer to 0 = better match).
-	// Examples: -0.5 = excellent match, -2.0 = good match, -10.0 = partial match.
+	// Rank is the bm25 relevance score (negative value, lower = better match).
+	// Examples: -5.0 = excellent match, -2.0 = good match, -0.5 = partial match.
 	Rank float64 `json:"rank,omitempty"`
 }
 
@@ -79,7 +79,7 @@ type GrepMessageResult struct {
 	Snippet        string  `json:"snippet"`
 	Role           string  `json:"role"`
 	ConversationID int64   `json:"conversationId"`
-	Rank           float64 `json:"rank,omitempty"` // Relevance score (lower = better match)
+	Rank           float64 `json:"rank,omitempty"` // Relevance score (more negative = better match)
 }
 
 // ExpandMessagesResult contains expanded messages.

pkg/seahorse/tool_grep.go

@@ -56,8 +56,8 @@ Returns:
   "hint": "No matches. Try: %keyword% for fuzzy search"
 }
 
-Rank field (FTS5 mode only): bm25 relevance score, negative value where closer to 0 = better match.
-Examples: -0.5=excellent, -2=good, -5=partial, -10=weak. LIKE mode (%pattern%) has no rank.
+Rank field (FTS5 mode only): bm25 relevance score, negative value where more negative = higher relevance.
+Examples: -5=excellent, -2=good, -0.5=partial. LIKE mode (%pattern%) has no rank.
 
 Examples:
   {"pattern": "authentication"}