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/components/MarkdownArtifactPane.tsx

@@ -8,11 +8,12 @@
 // the matching message_id. This keeps sessions.workspace_panes jsonb small
 // and the assistant message row remains the single source of truth.
 import { useEffect, useState } from 'react';
-import { Check, Copy, Download, X } from 'lucide-react';
 import { toast } from 'sonner';
 import { api } from '@/api/client';
 import type { MarkdownArtifactState } from '@/api/types';
 import { MarkdownRenderer } from './MarkdownRenderer';
+import { ArtifactPaneHeader } from './ArtifactPaneHeader';
+import { useArtifactDownload } from '@/hooks/useArtifactDownload';
 
 interface Props {
   chatId: string;
@@ -22,9 +23,9 @@ interface Props {
 
 export function MarkdownArtifactPane({ chatId, state, onClose }: Props) {
   const [justCopied, setJustCopied] = useState(false);
-  const [downloading, setDownloading] = useState(false);
   const [content, setContent] = useState<string | null>(null);
   const [loadError, setLoadError] = useState<string | null>(null);
+  const { downloading, download } = useArtifactDownload(chatId, state.message_id, 'md');
 
   useEffect(() => {
     let cancelled = false;
@@ -64,65 +65,18 @@ export function MarkdownArtifactPane({ chatId, state, onClose }: Props) {
     }
   }
 
-  async function download() {
-    if (downloading) return;
-    setDownloading(true);
-    try {
-      const { url, path } = await api.messages.downloadArtifact(
-        chatId,
-        state.message_id,
-        'md',
-      );
-      // Trigger browser download from the returned URL. The endpoint stamps
-      // Content-Disposition: attachment so the click lands as a save.
-      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 (
     <div className="flex flex-col h-full min-h-0">
-      <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={state.title}>
-          {state.title || 'Markdown artifact'}
-        </span>
-        <button
-          type="button"
-          onClick={() => void copy()}
-          disabled={content === null}
-          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 markdown source"
-          title="Copy"
-        >
-          {justCopied ? <Check size={12} /> : <Copy size={12} />}
-        </button>
-        <button
-          type="button"
-          onClick={() => void download()}
-          disabled={downloading || content === null}
-          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 markdown"
-          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>
+      <ArtifactPaneHeader
+        title={state.title}
+        defaultTitle="Markdown artifact"
+        onDownload={() => void download()}
+        downloadDisabled={downloading || content === null}
+        onClose={onClose}
+        onCopy={() => void copy()}
+        justCopied={justCopied}
+        copyDisabled={content === null}
+      />
       <div className="flex-1 min-h-0 overflow-auto px-4 py-3 text-sm">
         {loadError ? (
           <div className="text-destructive">Failed to load: {loadError}</div>