NM-283: fix missing endpoint-ip6 flag name and restrict MTU flag to push cmd

[abhishek9686]

The EndpointIP6 field was not initialized in registerFlags, causing the --endpoint-ip6 / -E flag to render as an unnamed flag. Also remove the MTU flag from join and register commands since MTU should only be changed via push after the host is already enrolled.

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• My changes affect only 10 files or less.
• I have performed a self-review of my code and tested it.
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[tenki-reviewer]

Tenki Code Review - Complete

Files Reviewed: 2
Findings: 0

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This PR cleans up flag registration by removing the --mtu flag from join/register commands and adding a missing EndpointIP6 struct field initializer. One inconsistency was found: push exposes EndpointIP but not EndpointIP6, leaving IPv6 endpoint updates inaccessible from that command.

Files Reviewed (2 files)

cmd/join.go
cmd/register.go