refactor: route raw Bun.file/write/hash/which/spawn through runtime shims

Eliminates 19 unguarded `Bun.*` runtime call sites in the plugin bundle
that crashed with `ReferenceError: Bun is not defined` under Electron.

Per-tool-call hot paths (executed on every Read/Edit):
- src/tools/hashline-edit/hash-computation.ts: Bun.hash.xxHash32 → bunHashXxh32
- src/tools/hashline-edit/hashline-edit-executor.ts: 8 sites via bunFile/bunWrite
- src/hooks/hashline-read-enhancer/hook.ts: Bun.file → bunFile
- src/hooks/hashline-edit-diff-enhancer/hook.ts: 2 sites via bunFile

Plugin-load paths:
- src/hooks/claude-code-hooks/config.ts and config-loader.ts: Bun.file → bunFile
- src/features/claude-code-mcp-loader/loader.ts: Bun.file → bunFile
- src/features/claude-code-plugin-loader/mcp-server-loader.ts: Bun.file → bunFile
- src/features/team-mode/deps.ts: Bun.spawn → spawn shim
- src/hooks/session-notification-utils.ts: Bun.which → bunWhich, also drops
  the bare `declare const Bun` ambient declaration
- src/shared/binary-downloader.ts: Bun.write → bunWrite

Pure mechanical API swaps. No control-flow or signature changes.

src/features/claude-code-plugin-loader/mcp-server-loader.ts

@@ -7,6 +7,7 @@ import type { ClaudeCodeMcpConfig } from "../claude-code-mcp-loader/types"
 import { log } from "../../shared/logger"
 import type { LoadedPlugin } from "./types"
 import { resolvePluginPaths } from "./plugin-path-resolver"
+import { bunFile } from "../../shared/bun-file-shim"
 
 export async function loadPluginMcpServers(
   plugins: LoadedPlugin[],
@@ -18,7 +19,7 @@ export async function loadPluginMcpServers(
     if (!plugin.mcpPath || !existsSync(plugin.mcpPath)) continue
 
     try {
-      const content = await Bun.file(plugin.mcpPath).text()
+      const content = await bunFile(plugin.mcpPath).text()
       let config = JSON.parse(content) as ClaudeCodeMcpConfig
 
       config = resolvePluginPaths(config, plugin.installPath)