chore: cleanup sprint — dead CSS, dedup helpers, handler refactor

Remove ~126 lines of orphaned CSS from tile slim-down and old detail
layout. Consolidate 4 duplicate duration formatters into shared
elapsed()/fmtElapsed() helpers. Break 160-line Result handler into
focused sub-functions. Implement real Hub.Shutdown() (was a no-op).
Standardize agent error responses to JSON. Replace panic() in router
init with error return. Extract magic numbers as named constants.

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

internal/orchestrator/dispatcher.go

@@ -18,7 +18,10 @@ import (
 // doesn't block dispatch. Used by the StartRun preflight and the
 // dispatcher itself — both must agree or the operator's click-time
 // validation wouldn't match the dispatch-time check.
-const HostHeartbeatStaleAfter = 60 * time.Second
+const (
+	HostHeartbeatStaleAfter = 60 * time.Second
+	dispatchTickInterval    = 2 * time.Second
+)
 
 // Dispatcher picks Queued runs off the DB and drives them to
 // WaitingReboot — the happy path is heartbeat-first: we transition and
@@ -76,7 +79,7 @@ func (d *Dispatcher) Stop() {
 }
 
 func (d *Dispatcher) loop(ctx context.Context) {
-	t := time.NewTicker(2 * time.Second)
+	t := time.NewTicker(dispatchTickInterval)
 	defer t.Stop()
 	for {
 		select {