Bump rc.21 — GitHub-as-transport pivot

WebRTC stripped, GitHub HTTPS as the always-online transport. Sovereignty
pattern: every peer has their own GitHub repo per DreamNode (origin =
own outbox, peer remotes = senders' outboxes, fetch-only). UUIDs
preserve canonical identity across transport.

Validated end-to-end Mac (Alice/ProjectLiminality) → Windows
(Bob/InterfaceGuy):
- Create + Share Changes propagates DreamNodes between peers
- Cylinder1 with Square + Circle submodules clones recursively
- Parent-origin transitivity resolves bare interbrain://<uuid> URLs
- Sovereign clones of submodules land at vault root (visible in
  holarchy nav, clickable from DreamSong canvases)
- Cherry-pick beacon flow accepts supermodule relationships across peers
- First-run flow installs gh CLI + wires git credential helper
  automatically — no manual auth ceremony on either platform

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: ProjectLiminality

desktop/src-tauri/Cargo.lock

@@ -1,6 +1,6 @@
 [package]
 name = "interbrain-desktop"
-version = "0.16.0-rc.20"
+version = "0.16.0-rc.21"
 description = "InterBrain — knowledge gardening companion"
 authors = ["Project Liminality"]
 license = "AGPL-3.0"

desktop/src-tauri/Cargo.toml

@@ -1,7 +1,7 @@
 {
   "$schema": "https://schema.tauri.app/config/2",
   "productName": "InterBrain",
-  "version": "0.16.0-rc.20",
+  "version": "0.16.0-rc.21",
   "identifier": "org.projectliminality.interbrain",
   "build": {
     "beforeDevCommand": "npm run dev",