refactor: codebase audit cleanup — dead code, dedup, module splits

Multi-agent audit + aggressive cleanup across server/web/coder/booterm,
delivered behind a DEFER discipline so none of the in-flight files were
touched. Removes dead code/deps/columns, dedups server + coder helpers,
and splits the oversized modules (tools.ts, opencode-server.ts,
sentinel-summaries, turn.ts, TerminalPane.tsx) behind stable contracts.
Adds 78 parity/unit tests (server 587, coder 323); fixes two latent bugs
(ChatPane queue keys, FileViewerOverlay blank-line parity).

Intended tag: v2.7.12-audit-cleanup.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/web/src/components/ArtifactPaneHeader.tsx

@@ -0,0 +1,63 @@
+import { Check, Copy, Download, X } from 'lucide-react';
+
+interface Props {
+  title: string;
+  defaultTitle: string;
+  onDownload: () => void;
+  downloadDisabled: boolean;
+  onClose: () => void;
+  // Optional copy button (Markdown pane only; HTML pane omits it).
+  onCopy?: () => void;
+  justCopied?: boolean;
+  copyDisabled?: boolean;
+}
+
+export function ArtifactPaneHeader({
+  title,
+  defaultTitle,
+  onDownload,
+  downloadDisabled,
+  onClose,
+  onCopy,
+  justCopied,
+  copyDisabled,
+}: Props) {
+  return (
+    <div className="flex items-center gap-2 border-b border-border bg-muted/30 px-2 py-1 shrink-0">
+      <span className="text-xs text-muted-foreground truncate flex-1" title={title}>
+        {title || defaultTitle}
+      </span>
+      {onCopy && (
+        <button
+          type="button"
+          onClick={onCopy}
+          disabled={copyDisabled}
+          className="inline-flex items-center justify-center size-5 rounded text-muted-foreground hover:bg-muted hover:text-foreground disabled:opacity-40 max-md:min-h-[44px] max-md:min-w-[44px]"
+          aria-label="Copy source"
+          title="Copy"
+        >
+          {justCopied ? <Check size={12} /> : <Copy size={12} />}
+        </button>
+      )}
+      <button
+        type="button"
+        onClick={onDownload}
+        disabled={downloadDisabled}
+        className="inline-flex items-center justify-center size-5 rounded text-muted-foreground hover:bg-muted hover:text-foreground disabled:opacity-40 max-md:min-h-[44px] max-md:min-w-[44px]"
+        aria-label="Download"
+        title="Download"
+      >
+        <Download size={12} />
+      </button>
+      <button
+        type="button"
+        onClick={onClose}
+        className="inline-flex items-center justify-center size-5 rounded text-muted-foreground hover:bg-muted hover:text-foreground max-md:min-h-[44px] max-md:min-w-[44px]"
+        aria-label="Close artifact pane"
+        title="Close"
+      >
+        <X size={12} />
+      </button>
+    </div>
+  );
+}