perf: Optimize SQL queries and reduce allocations in hot paths

Change detection queries (embedding/change_detector.rs):
- Replace triple-EXISTS subquery pattern with LEFT JOIN + NULL check
- SQLite now scans embedding_metadata once instead of three times
- Semantically identical: returns docs needing embedding when no
  embedding exists, hash changed, or config mismatch

Count queries (cli/commands/count.rs):
- Consolidate 3 separate COUNT queries for issues into single query
  using conditional aggregation (CASE WHEN state = 'x' THEN 1)
- Same optimization for MRs: 5 queries reduced to 1

Search filter queries (search/filters.rs):
- Replace N separate EXISTS clauses for label filtering with single
  IN() clause with COUNT/GROUP BY HAVING pattern
- For multi-label AND queries, this reduces N subqueries to 1

FTS tokenization (search/fts.rs):
- Replace collect-into-Vec-then-join pattern with direct String building
- Pre-allocate capacity hint for result string

Discussion truncation (documents/truncation.rs):
- Calculate total length without allocating concatenated string first
- Only allocate full string when we know it fits within limit

Embedding pipeline (embedding/pipeline.rs):
- Add Vec::with_capacity hints for chunk work and cleared_docs hashset
- Reduces reallocations during embedding batch processing

Backoff calculation (core/backoff.rs):
- Replace unchecked addition with saturating_add to prevent overflow
- Add test case verifying overflow protection

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

src/search/fts.rs

@@ -23,22 +23,25 @@ pub fn to_fts_query(raw: &str, mode: FtsQueryMode) -> String {
                 return String::new();
             }
 
-            let tokens: Vec<String> = trimmed
-                .split_whitespace()
-                .map(|token| {
-                    if let Some(stem) = token.strip_suffix('*')
-                        && !stem.is_empty()
-                        && stem.chars().all(|c| c.is_alphanumeric() || c == '_')
-                    {
-                        let escaped = stem.replace('"', "\"\"");
-                        return format!("\"{}\"*", escaped);
-                    }
-                    let escaped = token.replace('"', "\"\"");
-                    format!("\"{}\"", escaped)
-                })
-                .collect();
-
-            tokens.join(" ")
+            let mut result = String::with_capacity(trimmed.len() + 20);
+            for (i, token) in trimmed.split_whitespace().enumerate() {
+                if i > 0 {
+                    result.push(' ');
+                }
+                if let Some(stem) = token.strip_suffix('*')
+                    && !stem.is_empty()
+                    && stem.chars().all(|c| c.is_alphanumeric() || c == '_')
+                {
+                    result.push('"');
+                    result.push_str(&stem.replace('"', "\"\""));
+                    result.push_str("\"*");
+                } else {
+                    result.push('"');
+                    result.push_str(&token.replace('"', "\"\""));
+                    result.push('"');
+                }
+            }
+            result
         }
     }
 }