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/web/static/app.css

@@ -140,58 +140,6 @@ button.danger:hover { background: rgba(229,100,102,.1); }
 .tile-last-seen.stale::before { background: var(--warn); }
 .tile-last-seen.offline::before { background: var(--text-dim); opacity: .5; }
 
-.tile-meta { display: grid; grid-template-columns: 1fr 1fr; gap: 4px 16px; margin: 0; font-size: 13px; }
-.tile-meta div { display: flex; justify-content: space-between; align-items: baseline; }
-.tile-meta dt { color: var(--text-dim); }
-.tile-meta dd { margin: 0; font-family: var(--mono); }
-
-.tile-actions { display: flex; gap: 8px; }
-.tile-actions .inline { margin: 0; flex: 0; }
-
-.tile-meta dd.bad { color: var(--danger); }
-
-.tile-hold {
-  background: rgba(229,100,102,.08);
-  border: 1px solid rgba(229,100,102,.35);
-  border-radius: var(--radius);
-  padding: 8px 10px;
-  display: flex;
-  flex-direction: column;
-  gap: 4px;
-}
-.tile-hold .hold-title {
-  font-size: 12px;
-  color: var(--danger);
-  text-transform: uppercase;
-  letter-spacing: .5px;
-}
-.tile-hold .hold-ssh {
-  font-family: var(--mono);
-  font-size: 12px;
-  color: var(--text);
-  word-break: break-all;
-  user-select: all;
-}
-
-.tile-log {
-  background: #0b0d12;
-  border: 1px solid var(--border);
-  border-radius: var(--radius);
-  padding: 8px 10px;
-  font-family: var(--mono);
-  font-size: 12px;
-  color: var(--text-dim);
-  max-height: 160px;
-  overflow-y: auto;
-  display: flex;
-  flex-direction: column;
-  gap: 2px;
-}
-.tile-log:empty { display: none; }
-.tile-log .log-line { white-space: pre-wrap; }
-.tile-log .log-warn { color: var(--warn); }
-.tile-log .log-error { color: var(--danger); }
-
 .tile-fail { border-color: rgba(229,100,102,.6); }
 .tile-pass { border-color: rgba(53,194,123,.5); }
 .tile-active { border-color: var(--accent); }
@@ -314,7 +262,6 @@ body.bare main { max-width: none; }
 .detail-summary.tile-active { border-color: var(--accent); }
 .detail-summary-head { display: flex; justify-content: space-between; align-items: baseline; gap: 16px; flex-wrap: wrap; }
 .detail-name { margin: 0; font-size: 22px; }
-.detail-status-row { display: flex; align-items: center; gap: 12px; }
 .detail-meta {
   display: grid;
   grid-template-columns: repeat(auto-fit, minmax(180px, 1fr));
@@ -339,11 +286,6 @@ body.bare main { max-width: none; }
 .detail-section details[open] > summary::before { transform: rotate(90deg); }
 .detail-section details > summary h2 { margin: 0; }
 
-.detail-hold {
-  background: rgba(229,100,102,.08);
-  border-color: rgba(229,100,102,.35);
-}
-.detail-hold h2 { color: var(--danger); }
 .hold-ssh {
   font-family: var(--mono);
   font-size: 13px;
@@ -360,25 +302,6 @@ body.bare main { max-width: none; }
 .detail-actions-row { display: flex; flex-wrap: wrap; gap: 10px; }
 .detail-actions-row .inline { margin: 0; }
 
-.detail-log {
-  background: #0b0d12;
-  border: 1px solid var(--border);
-  border-radius: var(--radius);
-  padding: 10px 12px;
-  font-family: var(--mono);
-  font-size: 12px;
-  color: var(--text-dim);
-  max-height: 500px;
-  overflow-y: auto;
-  display: flex;
-  flex-direction: column;
-  gap: 2px;
-}
-.detail-log:empty::before { content: "(no log output yet)"; color: var(--text-dim); opacity: .5; }
-.detail-log .log-line { white-space: pre-wrap; }
-.detail-log .log-warn { color: var(--warn); }
-.detail-log .log-error { color: var(--danger); }
-
 /* ===== Log tabs (CSS-only radio switch) ===== */
 /* Radios are visually hidden but still functional: checked state is read
    by sibling selectors below to flip the active label + pane. */
@@ -564,37 +487,6 @@ body.bare main { max-width: none; }
   50%       { box-shadow: 0 0 0 8px rgba(60,130,246,0); }
 }
 
-/* ===== Host detail v2 — GitHub-Actions-style layout ===== */
-
-.detail-v2 { gap: 12px; }
-
-.host-meta-drawer {
-  background: var(--bg-elev);
-  border: 1px solid var(--border);
-  border-radius: var(--radius);
-  padding: 8px 16px;
-}
-.host-meta-drawer > summary {
-  list-style: none;
-  cursor: pointer;
-  display: flex;
-  align-items: center;
-  gap: 16px;
-  font-size: 13px;
-  color: var(--text-dim);
-  padding: 4px 0;
-}
-.host-meta-drawer > summary::before {
-  content: "▸";
-  color: var(--text-dim);
-  font-size: 11px;
-  transition: transform .1s ease;
-}
-.host-meta-drawer[open] > summary::before { transform: rotate(90deg); }
-.host-meta-drawer .meta-summary-label { color: var(--text); font-weight: 600; }
-.host-meta-drawer .meta-summary-mac { font-family: var(--mono); margin-left: auto; }
-.host-meta-drawer[open] > summary { margin-bottom: 12px; border-bottom: 1px solid var(--border); padding-bottom: 8px; }
-
 .run-header {
   background: var(--bg-elev);
   border: 1px solid var(--border);
@@ -673,25 +565,7 @@ body.bare main { max-width: none; }
 }
 .detail-hold-placeholder { display: none; }
 
-.detail-body {
-  display: grid;
-  grid-template-columns: 1fr 260px;
-  gap: 16px;
-  align-items: start;
-}
-@media (max-width: 900px) {
-  .detail-body { grid-template-columns: 1fr; }
-}
 .active-step-pane { display: flex; flex-direction: column; gap: 8px; }
-.detail-empty {
-  padding: 24px;
-  background: var(--bg-elev);
-  border: 1px dashed var(--border);
-  border-radius: var(--radius);
-  color: var(--text-dim);
-  text-align: center;
-}
-
 .step {
   background: var(--bg-elev);
   border: 1px solid var(--border);