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/config.rs

@@ -69,7 +69,6 @@ pub struct GlobalConfig {
     pub shell_env: HashMap<String, String>,
     pub cache_version: u32,
     pub drop_strategy: String,
-    pub breakpoint_hysteresis: u16,
 }
 
 impl Default for GlobalConfig {
@@ -80,7 +79,7 @@ impl Default for GlobalConfig {
             vcs: "auto".into(),
             width: None,
             width_margin: 4,
-            cache_dir: "/tmp/claude-sl-{session_id}".into(),
+            cache_dir: "/tmp/claude-sl-{session_id}-{cache_version}-{config_hash}".into(),
             cache_gc_days: 7,
             cache_gc_interval_hours: 24,
             cache_ttl_jitter_pct: 10,
@@ -100,7 +99,6 @@ impl Default for GlobalConfig {
             shell_env: HashMap::new(),
             cache_version: 1,
             drop_strategy: "tiered".into(),
-            breakpoint_hysteresis: 2,
         }
     }
 }