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.

apps/web/src/hooks/useArtifactDownload.ts

@@ -0,0 +1,26 @@
+import { useState } from 'react';
+import { toast } from 'sonner';
+import { api } from '@/api/client';
+
+export function useArtifactDownload(chatId: string, messageId: string, format: 'md' | 'html') {
+  const [downloading, setDownloading] = useState(false);
+
+  async function download() {
+    if (downloading) return;
+    setDownloading(true);
+    try {
+      const { url, path } = await api.messages.downloadArtifact(chatId, messageId, format);
+      const a = document.createElement('a');
+      a.href = url;
+      a.rel = 'noopener';
+      a.click();
+      toast.success(`Saved to ${path}`);
+    } catch (err) {
+      toast.error(err instanceof Error ? err.message : 'download failed');
+    } finally {
+      setDownloading(false);
+    }
+  }
+
+  return { downloading, download };
+}