fix(bun-spawn-shim): eliminate globalThis.Bun top-level destructures for Electron/Node compat

Root cause: bun build --target bun inlines top-level
  var { spawn } = globalThis.Bun;
for every file that contains 'import { spawn } from "bun"'. On Node/Electron
where globalThis.Bun is undefined, this crashes with
  Cannot destructure property 'spawn' of 'globalThis.Bun' as it is undefined.
26 source files had this import; the bundled output had 25 top-level destructures.

Fix:
- Add src/shared/bun-spawn-shim.ts: a thin wrapper that
  - delegates to Bun.spawn/spawnSync when globalThis.Bun is present (real Bun)
  - falls back to static ESM imports of node:child_process otherwise
  - uses static 'import { spawn } from "node:child_process"' so Bun bundler
    does NOT emit any globalThis.Bun destructures for this module
- Replace all 26 'from "bun"' spawn/spawnSync imports with relative paths to shim
- Replace 4 direct Bun.spawn() call sites with shim's spawn()
- Remove src/electron-compat.ts and script/prepend-electron-shim.ts (no longer needed)
- Update src/electron-compat.test.ts to assert 0 top-level globalThis.Bun destructures

Verification: grep -c '} = globalThis.Bun;' dist/index.js → 0 (was 25)
All 5921 tests pass (1 pre-existing timeout failure unrelated to this change).

Fixes #3797

src/tools/ast-grep/cli.ts

@@ -1,4 +1,4 @@
-import { spawn } from "bun"
+import { spawn } from "../../shared/bun-spawn-shim"
 import { existsSync } from "fs"
 import {
 	getSgCliPath,

src/tools/glob/cli.ts

@@ -1,5 +1,5 @@
 import { resolve } from "node:path"
-import { spawn } from "bun"
+import { spawn } from "../../shared/bun-spawn-shim"
 import {
   resolveGrepCli,
   type GrepBackend,

src/tools/grep/cli.ts

@@ -1,4 +1,4 @@
-import { spawn } from "bun"
+import { spawn } from "../../shared/bun-spawn-shim"
 import {
   resolveGrepCli,
   type ResolvedCli,

src/tools/hashline-edit/formatter-trigger.ts

@@ -1,5 +1,6 @@
 import path from "path"
 import { log } from "../../shared"
+import { spawn as bunSpawn } from "../../shared/bun-spawn-shim"
 
 interface FormatterConfig {
   disabled?: boolean
@@ -106,7 +107,7 @@ export async function runFormattersForFile(
     const cmd = buildFormatterCommand(formatter.command, filePath)
     try {
       log("[formatter-trigger] Running formatter", { command: cmd, file: filePath })
-      const proc = Bun.spawn(cmd, {
+      const proc = bunSpawn(cmd, {
         cwd: directory,
         env: { ...process.env, ...formatter.environment },
         stdout: "ignore",

src/tools/interactive-bash/tmux-path-resolver.ts

@@ -1,4 +1,4 @@
-import { spawn } from "bun"
+import { spawn } from "../../shared/bun-spawn-shim"
 
 let tmuxPath: string | null = null
 let initPromise: Promise<string | null> | null = null

src/tools/lsp/lsp-process.ts

@@ -1,4 +1,4 @@
-import { spawn as bunSpawn } from "bun"
+import { spawn as bunSpawn } from "../../shared/bun-spawn-shim"
 import { spawn as nodeSpawn, type ChildProcess } from "node:child_process"
 import { existsSync, statSync } from "fs"
 import { log } from "../../shared/logger"