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

internal/secrets/keychain.go

@@ -27,7 +27,7 @@ func NewKeychainStore(serviceName string) *KeychainStore {
 // without error.
 func (k *KeychainStore) Available() bool {
 	// Attempt a round-trip to verify the keychain is usable.
-	probeKey := "__nexus_keychain_probe__"
+	probeKey := "__link_keychain_probe__"
 	probeVal := "probe"
 
 	if err := keyring.Set(k.serviceName, probeKey, probeVal); err != nil {