Add fork notice, NOTICE file, README/CONTRIBUTING rewrite; drop HashiCorp logo assets

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -1,3 +1,17 @@
+# Threshold CHANGELOG
+
+> **Note:** Threshold is a community fork of HashiCorp Boundary Community Edition (CE), which was archived upstream on
+> 2026-10-09. Entries below this note are the upstream Boundary changelog and are kept unchanged for history.
+> Threshold-specific changes are listed under the "Threshold" heading, newest first.
+
+## Threshold
+
+### Unreleased
+
+* Renamed the CLI binary to `threshold` and rebranded user-visible text. `THRESHOLD_*` environment variables are now
+  preferred, with `BOUNDARY_*` honored as a fallback. The client cache directory is `~/.threshold` (falling back to an
+  existing `~/.boundary`), and stored tokens are written under new keyring names while old entries are still read.
+
 # Boundary CHANGELOG
 
 Canonical reference for changes, improvements, and bugfixes for Boundary.