Reorganize project structure: move Go files to internal/beads, docs to docs/

Amp-Thread-ID: https://ampcode.com/threads/T-7a71671d-dd5c-4c7c-b557-fa427fceb04f
Co-authored-by: Amp <amp@ampcode.com>

docs/ATTRIBUTION.md

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+# Attribution and Credits
+
+## beads-merge 3-Way Merge Algorithm
+
+The 3-way merge functionality in `internal/merge/` is based on **beads-merge** by **@neongreen**.
+
+- **Original Repository**: https://github.com/neongreen/mono/tree/main/beads-merge
+- **Author**: @neongreen (https://github.com/neongreen)
+- **Integration Discussion**: https://github.com/neongreen/mono/issues/240
+
+### What We Vendored
+
+The core merge algorithm from beads-merge has been adapted and integrated into bd:
+- Field-level 3-way merge logic
+- Issue identity matching (id + created_at + created_by)
+- Dependency and label merging with deduplication
+- Timestamp handling (max wins)
+- Deletion detection
+- Conflict marker generation
+
+### Changes Made
+
+- Adapted to use bd's `internal/types.Issue` instead of custom types
+- Integrated with bd's JSONL export/import system
+- Added support for bd-specific fields (Design, AcceptanceCriteria, etc.)
+- Exposed as `bd merge` CLI command and library API
+
+### License
+
+The original beads-merge code is licensed under the MIT License:
+
+```
+MIT License
+
+Copyright (c) 2025 Emily (@neongreen)
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+```
+
+### Thank You
+
+Special thanks to @neongreen for building beads-merge and graciously allowing us to integrate it into bd. This solves critical multi-workspace sync issues and makes beads much more robust for collaborative workflows.