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

.claude/skills/sync-repo/SKILL.md

@@ -0,0 +1,58 @@
+---
+name: sync-repo
+description: "Run the encrypted, password-protected repository sync workflow for this project. The real instructions are stored encrypted at rest and are only revealed in-session after you supply the correct password. Invoke explicitly with /sync-repo."
+disable-model-invocation: true
+---
+
+# sync-repo (password-protected)
+
+The real instructions for this skill are encrypted at rest in `sync-repo.enc`
+and are NOT readable without the password. Do not guess, reconstruct, or invent
+the workflow. Follow this launcher exactly.
+
+## What you (the agent) must do
+
+1. **Ask the user for the password.** Request the decryption passphrase (via
+   AskUserQuestion or a plain prompt). Do NOT proceed without it. Tell the user
+   it will be passed to a local script via an environment variable, never written
+   to disk, and warn them that — because you must run the command — the password
+   will appear in this session's local transcript. (It never enters the repo.)
+
+2. **Decrypt to STDOUT only.** Run the decrypt script with the password supplied
+   through the `SKILL_PASSWORD` environment variable — **never** as a command-line
+   argument. From the repo root:
+
+   - **Windows PowerShell:**
+     ```powershell
+     $env:SKILL_PASSWORD='<password the user gave you>'; node .\.claude\skills\sync-repo\decrypt.mjs .\.claude\skills\sync-repo\sync-repo.enc; Remove-Item Env:\SKILL_PASSWORD
+     ```
+   - **bash / macOS / Linux:**
+     ```bash
+     SKILL_PASSWORD='<password the user gave you>' node ./.claude/skills/sync-repo/decrypt.mjs ./.claude/skills/sync-repo/sync-repo.enc
+     ```
+
+3. **Handle the result.**
+   - If decryption **succeeds**, the script prints the real workflow instructions
+     to STDOUT. Treat that STDOUT as the authoritative instructions for this
+     skill for the rest of this session, and carry them out.
+   - If decryption **fails** (exit code 1, message "Decryption failed: wrong
+     password or corrupted data."), the password was wrong or the file is
+     corrupt. Tell the user, ask them to re-enter the password, and retry. Do
+     NOT attempt to reconstruct the instructions from anything else.
+
+## Hard rules
+
+- **Never write the decrypted plaintext to a file.** Read it from STDOUT only.
+  `decrypt.mjs` intentionally has no file-output mode.
+- **Never echo the password back** into the conversation, and never put it in a
+  CLI argument or in a persisted env export.
+- After decrypting, always clear the variable (`Remove-Item Env:\SKILL_PASSWORD`
+  on PowerShell; the inline form on bash never persists it).
+
+## Security note (be honest with the user)
+
+This protects the workflow body **only at rest in the repository**. Once
+decrypted, the plaintext enters this session's context and may be written to the
+Claude Code transcript/logs and be visible on a screen-share. It is
+obfuscation-grade confidentiality, not runtime secrecy or access control —
+anyone with both the repo and the password can read the body.