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/inference/tool-phase.ts

@@ -22,7 +22,7 @@ import type {
   InferenceContext,
   StreamResult,
   TurnArgs,
-} from './turn.js';
+} from './types.js';
 // v1.13.13: synthesis pipeline — replaces the immediate recursive turn when
 // any of this batch's tool calls is in SYNTHESIS_TOOLS. Falls through to
 // recursion on synthesis failure (timeout / model error). See module header