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/FileViewerOverlay.tsx

@@ -9,9 +9,7 @@ interface Props {
   path: string;
   content: string;
   lang: string | null;
-  projectId: string;
   onClose: () => void;
-  onNavigate: (path: string) => void;
 }
 
 const SHIKI_THEME = 'github-dark';
@@ -21,7 +19,11 @@ function splitShikiLines(html: string): string[] {
   if (!match) return [];
   const inner = match[1]!;
   const lines = inner.split(/(?=<span class="line">)/);
-  return lines.filter(l => l.trim().length > 0);
+  // Keep every line segment (including blank `<span class="line"></span>`
+  // entries) so lineHtmls.length matches the raw line count and line-number
+  // indexing stays in sync. The first element before the first line span is
+  // discarded (it's always empty or whitespace).
+  return lines.filter(l => l.startsWith('<span class="line">'));
 }
 
 function basename(path: string): string {