feat: Add bd merge command for git 3-way JSONL merging (bd-omx1)

- Implemented bd merge command with dual-mode operation:
  1. Git 3-way merge: bd merge <output> <base> <left> <right>
  2. Duplicate issue merge: bd merge <sources...> --into <target> (placeholder)
- Added MergeFiles wrapper to internal/merge package
- Command works without database when used as git merge driver
- Supports --debug flag for verbose output
- Exit code 0 for clean merge, 1 for conflicts
- Handles deletions intelligently (delete-modify conflicts)
- Added proper MIT license attribution for @neongreen's beads-merge code
- Tests pass for git merge functionality

This enables git merge driver setup for .beads/beads.jsonl files.

internal/merge/merge_test.go

@@ -1,7 +1,11 @@
 // Package merge implements 3-way merge for beads JSONL files.
 //
 // This code is vendored from https://github.com/neongreen/mono/tree/main/beads-merge
-// Original author: @neongreen (https://github.com/neongreen)
+// Original author: Emily (@neongreen, https://github.com/neongreen)
+//
+// MIT License
+// Copyright (c) 2025 Emily
+// See ATTRIBUTION.md for full license text
 package merge
 
 import (