fix(core): strip BOM from AI-generated content (Issue #368)

[moizlatif7]

Summary

Fixes #368 by stripping UTF-8 BOMs from AI-generated content while preserving BOMs in existing files.

Problem

On Windows systems, OpenCode was adding UTF-8 BOMs (0xEF 0xBB 0xBF) to newly created files when AI-generated content contained a BOM character (\uFEFF). This broke Linux scripts because the shebang (#!/bin/bash) must be the very first bytes in the file.

Root cause: The logic was desiredBom = source.bom || next.bom

• source.bom: Does existing file have BOM? ✅ Correct to preserve
• next.bom: Does AI content have BOM? ❌ Should NOT trust

Solution

Changed BOM handling to: desiredBom = source.bom

This means:

• ✅ Existing files with BOMs keep their BOMs (preserves round-trips)
• ✅ New files never get BOMs (fixes Windows issue)
• ✅ AI-generated BOMs are stripped (fixes BOM in generated File Bug #368)
• ✅ Works on Windows, macOS, and Linux

Changes

Modified Files (7 changes across 6 files):

All changes include ANRCODE_CHANGE markers adjacent to modified code:

1. packages/core/src/file-mutation.ts:117 - Core V2 write logic
2. packages/core/src/patch.ts:80 - Patch derivation
3. packages/core/src/tool/edit.ts:195 - Core edit tool
4. packages/opencode/src/tool/write.ts:49 - Write tool
5. packages/opencode/src/tool/edit.ts:98, 134 - Edit tool (2 locations)
6. packages/opencode/src/patch/index.ts:338 - Patch application

Test Updates:

• Updated existing test: "preserves BOM from existing files but strips BOM from new content"
• Added new test: "strips BOM from AI-generated content in new files (Issue BOM in generated File Bug #368)"

Testing

✅ All tests pass:

• 1075 core tests pass (0 fail)
• 335 tool tests pass (0 fail)
• All BOM-specific tests pass
• Issue BOM in generated File Bug #368 test case passes

✅ Acceptance Criteria Met:

• New files with BOM in AI content → NO BOM written
• Existing files with BOM → BOM preserved
• Existing files without BOM → NO BOM added
• Linux scripts remain executable

ANR Workflow

This fix was implemented using proper ANR workflow:

• ✅ ANR Researcher analyzed and created implementation plan
• ✅ ANR Implementor applied changes with ANRCODE_CHANGE markers
• ✅ ANR Tester validated all changes and tests
• ✅ All markers placed adjacent to modified code per AGENTS.md

Verification

To verify the fix works:

1. Create a new file with AI-generated content containing BOM
2. Verify the file does NOT have BOM bytes at the start
3. Existing files with BOMs should preserve them on edit

Closes #368

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