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/services/codecontext_client.ts

@@ -113,7 +113,7 @@ export async function callCodecontext(
   fetcher: typeof fetch = fetch,
 ): Promise<CodecontextResponse> {
   // Step 1: realpath the project root, then realpath the requested target_dir
-  // (defaulting to projectPath when the caller didn't pass one — the 8 wrappers
+  // (defaulting to projectPath when the caller didn't pass one — the 12 wrappers
   // never pass target_dir; tests can override). A non-existent target_dir
   // throws before we hit the network so the model gets a sharp error.
   const resolvedProject = await realpath(req.projectPath);