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/team-mode/deps.ts

@@ -1,4 +1,5 @@
 import type { TeamModeConfig } from "../../config/schema/team-mode"
+import { spawn } from "../../shared/bun-spawn-shim"
 
 export interface TeamModeDependencyReport {
   tmuxAvailable: boolean
@@ -20,7 +21,7 @@ export async function checkTeamModeDependencies(
 
 async function probeBinary(cmd: string, args: string[]): Promise<boolean> {
   try {
-    const proc = Bun.spawn({ cmd: [cmd, ...args], stdout: "pipe", stderr: "pipe" })
+    const proc = spawn({ cmd: [cmd, ...args], stdout: "pipe", stderr: "pipe" })
     const code = await proc.exited
     return code === 0
   } catch {