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/server/src/utils/string-utils.ts

@@ -0,0 +1,9 @@
+// Strips a matching pair of leading/trailing single or double quotes from a
+// YAML value string produced by the minimal frontmatter parsers in agents.ts
+// and skills.ts. Returns the string unchanged when no matching pair is found.
+export function stripQuotes(s: string): string {
+  if (s.length >= 2 && (s[0] === '"' || s[0] === "'") && s[0] === s[s.length - 1]) {
+    return s.slice(1, -1);
+  }
+  return s;
+}