feat: implement remaining PRD features (10 beads)

Complete the PRD feature set with shell gating pipeline, cache
improvements, layout enhancements, and diagnostics:

- Shell: exec_gated with allowlist/denylist, circuit breaker, env merge
- Shell: parallel prefetch via std::thread::scope for cold renders
- Cache: TTL jitter (FNV-1a), config hash namespace, garbage collection
- Cache: diagnostic tracking (hit/miss, age) for dump-state
- Layout: gradual drop strategy (one-by-one vs tiered)
- Layout: render budget timer with graceful priority-based degradation
- Layout: breakpoint hysteresis to prevent preset toggling
- Width: detection source tracking for diagnostics
- CLI: --no-cache, --no-shell, --clear-cache, env var overrides
- Diagnostics: enhanced --dump-state with section timing and cache stats

Closes: bd-3oy, bd-62g, bd-khk, bd-3q1, bd-ywx, bd-3l2,
        bd-2vm, bd-1if, bd-2qr, bd-30o, bd-3ax, bd-3uw, bd-4b1

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

src/section/beads.rs

@@ -13,18 +13,24 @@ pub fn render(ctx: &RenderContext) -> Option<SectionOutput> {
         return None;
     }
 
+    // --no-shell: serve stale cache only
+    if ctx.no_shell {
+        return render_from_cache(ctx, ctx.cache.get_stale("beads_summary")?);
+    }
+
     let ttl = Duration::from_secs(ctx.config.sections.beads.ttl);
-    let timeout = Duration::from_millis(200);
 
     let cached = ctx.cache.get("beads_summary", ttl);
     let summary = cached.or_else(|| {
-        // Run br ready to get count of ready items
-        let out = shell::exec_with_timeout(
-            "br",
-            &["ready", "--json"],
-            Some(ctx.project_dir.to_str()?),
-            timeout,
-        )?;
+        // Use prefetched result if available, otherwise exec
+        let out = ctx.shell_results.get("beads").cloned().unwrap_or_else(|| {
+            shell::exec_gated(
+                ctx.shell_config,
+                "br",
+                &["ready", "--json"],
+                Some(ctx.project_dir.to_str()?),
+            )
+        })?;
         // Count JSON array items (simple: count opening braces at indent level 1)
         let count = out.matches("\"id\"").count();
         let summary = format!("{count}");
@@ -46,3 +52,17 @@ pub fn render(ctx: &RenderContext) -> Option<SectionOutput> {
 
     Some(SectionOutput { raw, ansi })
 }
+
+fn render_from_cache(ctx: &RenderContext, summary: String) -> Option<SectionOutput> {
+    let count: usize = summary.trim().parse().unwrap_or(0);
+    if count == 0 {
+        return None;
+    }
+    let raw = format!("{count} ready");
+    let ansi = if ctx.color_enabled {
+        format!("{}{raw}{}", color::DIM, color::RESET)
+    } else {
+        raw.clone()
+    };
+    Some(SectionOutput { raw, ansi })
+}