fix: 6 bugs found during fresh-eyes code review

1. cache: record diagnostic miss before early-return on modified()/duration_since() failure
2. dump-state: resolve all cache_dir template vars ({cache_version}, {config_hash})
3. config: remove dead breakpoint_hysteresis field from GlobalConfig (breakpoints.hysteresis is used)
4. config: align Rust Default cache_dir with defaults.json template
5. vcs: apply branch truncation in render_stale_cache (--no-shell path)
6. vcs: fix jj prefetch retry on already-failed command (flatten→unwrap_or_else)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/cache.rs

@@ -95,8 +95,20 @@ impl Cache {
                 return None;
             }
         };
-        let modified = meta.modified().ok()?;
-        let age = SystemTime::now().duration_since(modified).ok()?;
+        let modified = match meta.modified().ok() {
+            Some(m) => m,
+            None => {
+                self.record_diag(key, false, None);
+                return None;
+            }
+        };
+        let age = match SystemTime::now().duration_since(modified).ok() {
+            Some(a) => a,
+            None => {
+                self.record_diag(key, false, None);
+                return None;
+            }
+        };
         let age_ms = age.as_millis() as u64;
         let effective_ttl = self.jittered_ttl(key, ttl);
         if age < effective_ttl {