Rebrand: Nexus Terminal → Alchementary Link

Renames the product across the codebase: Go module path, config/data
paths, keychain service name, export file extension, build tooling,
docs, and UI text. Sidebar brand mark now reads ">_ AlchementaryLink"
with "Alchementary" in the connection-group gold accent colour, and
the window title is "Alchementary Link".

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019T8bJS8BCtY5GeSG2HBj1n

build/darwin/Info.plist

@@ -8,7 +8,7 @@
         <key>CFBundleExecutable</key>
         <string>{{.OutputFilename}}</string>
         <key>CFBundleIdentifier</key>
-        <string>com.andydixon.nexusterminal</string>
+        <string>com.alchementary.link</string>
         <key>CFBundleVersion</key>
         <string>{{.Info.ProductVersion}}</string>
         <key>CFBundleGetInfoString</key>