…rs + renderer wiring

Defines the type-safe IPC contract between renderer ↔ main, wires the
listed handlers in electron/main.ts (still skipped by default until the
electron binary lands), and switches the 4 list-y screens (Sessions /
Plugins / MCP / Skills) to call the real IPC with a graceful empty-list
fallback when window.deepcode.* isn't present.

  · packages/core/src/ipc/protocol.ts (NEW, ~160 lines)
    - IpcRequestMap — exhaustive per-channel { req, res } typing for
      app:version, creds:load/save, settings:load, sessions:list/resume,
      plugins:list/install/setEnabled, mcp:list, skills:list/body,
      agent:start/abort/approve/answer.
    - IpcEventMap — one-way events (agent:event for streamed turns,
      updater:update-downloaded).
    - AgentStreamEvent — union of agent loop event (text_delta /
      tool_use / tool_result / usage / turn_complete / error) +
      approval_request + ask_user + turn_done — keyed by turnId so the
      renderer can route multiple concurrent turns.
    - newTurnId / newQuestionId helpers.
    - 4 tests + re-export from core index.

  · apps/desktop/electron/preload.ts — full typed surface exposed to
    renderer via contextBridge. window.deepcode now has sessions, plugins,
    mcp, skills, agent + onUpdateDownloaded.

  · apps/desktop/src/types/global.d.ts — corresponding renderer-side
    declarations for window.deepcode.

  · apps/desktop/electron/main.ts — added handlers:
    sessions:list, plugins:list, mcp:list, skills:list, skills:body.
    (Agent loop handlers + install/setEnabled mutators land alongside
    the live agent streaming wiring in a follow-up; the protocol is
    pinned now so the renderer code is final.)

  · apps/desktop/src/screens/{Sessions,Plugins,MCPManager,Skills}.tsx —
    real IPC fetch with empty-list fallback for current pre-electron state.

Tests: core 445 → 449 (+4 ipc); total 508 → 512 passing. Build clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>