Add password-protected sync-repo skill with encrypted body

Adds a project-local /sync-repo skill whose real instructions are encrypted
at rest (AES-256-GCM + scrypt) in sync-repo.enc and only revealed in-session
after the user supplies the correct passphrase. Plaintext SYNC.md is gitignored
and never committed; *.enc is marked binary to protect the ciphertext bytes.

AI Assisted

.gitattributes

@@ -0,0 +1,4 @@
+# The encrypted sync-repo blob is base64 text but must never have its bytes
+# altered by line-ending normalization. Treat it as binary so autocrlf/eol
+# settings can never corrupt the ciphertext.
+*.enc binary