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

package.json

@@ -22,8 +22,7 @@
     "./schema.json": "./dist/oh-my-opencode.schema.json"
   },
   "scripts": {
-    "build": "bun build src/index.ts --outdir dist --target bun --format esm --external @ast-grep/napi --external zod && bun run build:prepend-shim && tsc --emitDeclarationOnly && bun build src/cli/index.ts --outdir dist/cli --target bun --format esm --external @ast-grep/napi && bun run build:schema",
-    "build:prepend-shim": "bun run script/prepend-electron-shim.ts",
+    "build": "bun build src/index.ts --outdir dist --target bun --format esm --external @ast-grep/napi --external zod && tsc --emitDeclarationOnly && bun build src/cli/index.ts --outdir dist/cli --target bun --format esm --external @ast-grep/napi && bun run build:schema",
     "build:all": "bun run build && bun run build:binaries",
     "build:binaries": "bun run script/build-binaries.ts",
     "build:schema": "bun run script/build-schema.ts",