chore: rename mach273 → machbuilds across docs and config

GitHub username changed from mach273 to machbuilds. GitHub auto-
redirects the old URL, but updating in-repo references explicitly
keeps everything grep-able and avoids silent drift.

Updated:
  - LICENSE — copyright holder
  - README.md — git clone URL in Quick Start
  - CHANGELOG.md — Unreleased + v0.1.0 compare/release links
  - CODE_OF_CONDUCT.md — Security Advisories link
  - SECURITY.md — Security Advisories link + maintainer profile link
  - docs/planning/nucleus.md — example slug in two code blocks

Local git remote also updated:
  origin → https://github.com/machbuilds/atom.git

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: machbuilds

CHANGELOG.md

@@ -30,5 +30,5 @@ First feature-complete release. atom is a project-starter template with cross-pr
 - Stack presets currently include `nextjs` only. Other stacks fall back to the generic scaffold and will land per-stack in v0.2.
 - Constitution generation is a TODO marker in the cheatsheet; v0.2 will wire `speckit-constitution` automatically.
 
-[Unreleased]: https://github.com/mach273/atom/compare/v0.1.0...HEAD
-[0.1.0]: https://github.com/mach273/atom/releases/tag/v0.1.0
+[Unreleased]: https://github.com/machbuilds/atom/compare/v0.1.0...HEAD
+[0.1.0]: https://github.com/machbuilds/atom/releases/tag/v0.1.0

CODE_OF_CONDUCT.md

@@ -31,7 +31,7 @@ This Code applies in all project spaces — issues, pull requests, discussions,
 
 ## Reporting
 
-To report unacceptable behaviour, open a private security advisory on the [atom repo](https://github.com/mach273/atom/security/advisories/new) or email the maintainer (see `SECURITY.md` for current contact). All complaints will be reviewed promptly and confidentially.
+To report unacceptable behaviour, open a private security advisory on the [atom repo](https://github.com/machbuilds/atom/security/advisories/new) or email the maintainer (see `SECURITY.md` for current contact). All complaints will be reviewed promptly and confidentially.
 
 ## Attribution
 

LICENSE

@@ -1,6 +1,6 @@
 MIT License
 
-Copyright (c) 2026 mach273
+Copyright (c) 2026 machbuilds
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal

README.md

@@ -53,7 +53,7 @@ Platform: macOS, Linux. Windows users need Git Bash, WSL, or similar for the she
 ## Quick start
 
 ```bash
-git clone https://github.com/mach273/atom.git my-project
+git clone https://github.com/machbuilds/atom.git my-project
 cd my-project
 atom-setup
 ```

SECURITY.md

@@ -15,8 +15,8 @@ If you discover a security vulnerability in atom, the three CLIs (`atom-setup`,
 
 Instead, use one of these private channels:
 
-1. **GitHub Security Advisories** (preferred). Open a private advisory at https://github.com/mach273/atom/security/advisories/new. The maintainer is notified directly and the disclosure stays embargoed until a fix lands.
-2. **Email**: send a description to the maintainer's contact email listed in the GitHub profile at https://github.com/mach273. Include "atom security" in the subject.
+1. **GitHub Security Advisories** (preferred). Open a private advisory at https://github.com/machbuilds/atom/security/advisories/new. The maintainer is notified directly and the disclosure stays embargoed until a fix lands.
+2. **Email**: send a description to the maintainer's contact email listed in the GitHub profile at https://github.com/machbuilds. Include "atom security" in the subject.
 
 Please include:
 

docs/planning/nucleus.md

@@ -53,7 +53,7 @@ Each entry is one line in `learnings.jsonl`:
   "id": "01HXY...",
   "ts": "2026-05-04T15:23:11Z",
   "schema_version": 1,
-  "project": "mach273-atom",
+  "project": "machbuilds-atom",
   "type": "architecture",
   "key": "nucleus-uses-jsonl-not-sqlite",
   "insight": "Chose JSONL over SQLite for ~/.nucleus because git-diffable, grep-able, and human-readable. SQLite would be faster but kills the cross-project sync story.",
@@ -132,7 +132,7 @@ nucleus search "..." --semantic              # reserved for future
 Default human-readable output:
 
 ```
-mach273-atom · pitfall · high · 2026-04-12
+machbuilds-atom · pitfall · high · 2026-04-12
   Auth token refresh races on parallel requests cause 401 storms.
   Cache the refresh promise; share it across concurrent calls.
   → src/auth/refresh.ts