cwc-makers: curl+tar fallback when git is missing

/maker-setup now falls back to GitHub's tarball endpoint when git isn't
on PATH, instead of detouring through a package-manager git install.
curl and tar ship with macOS, Linux, and Windows 10 1803+ out of the
box, so this is zero-install on every target platform — and a CwC
attendee just needs the files once to flash a device, not git history.

- maker-setup.md: git-clone fast path, curl|tar (Unix) / curl+tar+
  Rename-Item (PowerShell) fallback, normalizes the -main suffix
- m5-onboard/SKILL.md: drop git from required deps + per-OS git
  bootstrap block; keep Python bootstrap
- README: git now listed as optional

Linear: CC-1975

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

🏠 Remote-Dev: homespace

.claude-plugin/marketplace.json

@@ -2014,20 +2014,6 @@
       "source": "./external_plugins/terraform",
       "homepage": "https://github.com/anthropics/claude-plugins-public/tree/main/external_plugins/terraform"
     },
-    {
-      "name": "twilio-developer-kit",
-      "description": "Twilio Skills provide procedural knowledge for AI coding agents — which APIs to use, in what order, and what to avoid. Covers SMS, Voice, WhatsApp, Verify, SendGrid, Compliance, and 30+ products.",
-      "author": {
-        "name": "Twilio"
-      },
-      "category": "development",
-      "source": {
-        "source": "url",
-        "url": "https://github.com/twilio/ai.git",
-        "sha": "0713fb1f40b5e871cad4c1c99f603c812431692a"
-      },
-      "homepage": "https://www.twilio.com"
-    },
     {
       "name": "typescript-lsp",
       "description": "TypeScript/JavaScript language server for enhanced code intelligence",