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/documents/truncation.rs

@@ -58,9 +58,13 @@ pub fn truncate_discussion(notes: &[NoteContent], max_bytes: usize) -> Truncatio
     }
 
     let formatted: Vec<String> = notes.iter().map(format_note).collect();
-    let total: String = formatted.concat();
+    let total_len: usize = formatted.iter().map(|s| s.len()).sum();
 
-    if total.len() <= max_bytes {
+    if total_len <= max_bytes {
+        let mut total = String::with_capacity(total_len);
+        for s in &formatted {
+            total.push_str(s);
+        }
         return TruncationResult {
             content: total,
             is_truncated: false,
@@ -69,7 +73,7 @@ pub fn truncate_discussion(notes: &[NoteContent], max_bytes: usize) -> Truncatio
     }
 
     if notes.len() == 1 {
-        let truncated = truncate_utf8(&total, max_bytes.saturating_sub(11));
+        let truncated = truncate_utf8(&formatted[0], max_bytes.saturating_sub(11));
         let content = format!("{}[truncated]", truncated);
         return TruncationResult {
             content,