feat(builds): add /builds.html — build lifecycle hub (closes #1999)

New standalone page /builds.html consolidating rebuild queue, meta
inputs, and build log history into one place, with a new 'Builds' home
tile linking to it. Addresses mara's request: new sub-page with a new
home tile, all three items moved there.

Changes:
- builds.html: new page with three sub-tabs: R3BU1LD QU3U3, M3T4 1NPUTS,
  BUILD L0GS. Same minimal-chrome header + createTabStrip pattern as core
  and logs pages.
- builds.js: new bundle combining rebuild queue renderer (from core.js),
  meta inputs renderer (from core.js), rebuild-live-log renderer (from
  core.js), and build log history renderer (from logs.js). Deep-links to
  /builds.html?id=N#buildlogs. Count pill id: builds-tab-count-rebuild.
  BUILD L0GS tab lazy-loads on first activation.
- builds.css: @imports system-sections.css (rebuild queue + meta inputs +
  live-log styles) and logs.css (build-logs-* component styles).
- build.mjs: register builds.js, builds.css, builds.html.
- core.html: remove R3BU1LD QU3U3 + M3T4 1NPUTS tabs (now on builds.html).
  Default tab changes to K3PT ST4T3.
- core.js: remove renderMetaInputs, renderRebuildQueue + helpers,
  renderRebuildLiveLog + live-log state, elapsed-time tickers,
  updateRebuildCount, and the rebuild_queue/meta_inputs SSE handlers.
  Remove openBuildLogStream + util imports no longer needed.
- core.css: remove .rebuild-live-log-* rules (moved to system-sections.css
  so builds.css can share them via @import).
- system-sections.css: add .rebuild-live-log-* styles (moved from core.css);
  update comment to mention builds.html.
- logs.html: remove BUILD tab + pane (moved to builds.html).
- logs.js: remove fetchBuild(), fmtTs, fmtDuration, openBuildLogStream
  import, and rebuild_queue_changed SSE debounce. Default tab: 'agent'.
  SSE stream retained for audit_entry_added live-appends.
- index.html: add Builds tile (🔨, rebuild queue · meta inputs · build
  logs); update Core tile desc to 'kept state · container load'; update
  Logs tile desc to remove 'build'.

frontend/packages/dashboard/src/index.html

@@ -51,12 +51,20 @@
         <span class="home-tile-desc">live all-agents message firehose</span>
       </a>
 
+      <a class="home-tile" href="/builds.html">
+        <span class="home-tile-head">
+          <span class="home-tile-icon" aria-hidden="true">🔨</span>
+          <span class="home-tile-label">Builds</span>
+        </span>
+        <span class="home-tile-desc">rebuild queue · meta inputs · build logs</span>
+      </a>
+
       <a class="home-tile" href="/logs.html">
         <span class="home-tile-head">
           <span class="home-tile-icon" aria-hidden="true">📜</span>
           <span class="home-tile-label">Logs</span>
         </span>
-        <span class="home-tile-desc">build · agent · system logs · privileged-action audit</span>
+        <span class="home-tile-desc">agent · system logs · privileged-action audit</span>
       </a>
 
       <a class="home-tile" href="/stats.html">
@@ -80,7 +88,7 @@
           <span class="home-tile-icon" aria-hidden="true">⬡</span>
           <span class="home-tile-label">Core</span>
         </span>
-        <span class="home-tile-desc">rebuild queue · meta inputs · kept state · container load</span>
+        <span class="home-tile-desc">kept state · container load</span>
       </a>
 
       <a class="home-tile" href="/matrix-accounts.html">