Parent epic: #84

Teach skillserver CLI how to validate, publish, and provide unopinionated sync primitives for NetClaw-compatible sub-agent definitions.

Tasks:

• Add sub-agent validation command or extend lint command.
• Add publish command for a single sub-agent .md file.
• Add duplicate handling and force behavior.
• Add dry-run and verbose output.
• Add destination-agnostic sync behavior that mirrors the existing skill sync model.
• Keep target-client destinations and format conversion outside the server protocol.
• Update CLI README.

Acceptance criteria:

• CLI validates required and optional fields consistently with the server.
• CLI publishes valid sub-agent definitions.
• CLI reports actionable errors for invalid files.
• CLI sync primitives can be reused by adapters for OpenCode or other non-NetClaw clients.
• CLI behavior does not hard-code NetClaw application paths for generic sync operations.