[deckhouse-cli] Merge "cli" and "plugins" command into d8 "dist"

[Glitchy-Sheep]

Summary

d8 cli (self-update) and d8 plugins (plugin management) were two halves of one job: managing the installed d8 binary and its plugins.

This PR merges them into a single entry point, d8 dist (the "distribution"), and adds a root summary.

Command tree

Before (both trees removed):

d8 cli
├── check
├── update
├── use <version>
└── versions   (alias: list)

d8 plugins                   (hidden)
├── list
├── versions
├── contract
├── install
├── update (+ all)
└── remove

After:

d8 dist                      # NEW: summary - d8 version, plugins, what is outdated
├── check                    # is a newer d8 available
├── update                   # update the d8 binary
├── use <version>            # switch / roll back the d8 binary
├── versions   (alias: list) # d8 versions published in the registry
└── plugins                  # the plugins subtree, unchanged (still hidden)
    ├── list
    ├── versions
    ├── contract
    ├── install
    ├── update (+ all)
    └── remove

What changed

• The four self-update commands moved from internal/selfupdate/cmd to a new internal/dist/cmd package, one file per command.
• The plugins subtree is mounted under d8 dist as is; it stays hidden while the plugin ecosystem rolls out.
• Cluster flags (--kubeconfig, --context, --rpp-*) moved to the dist root; the whole tree inherits them. The plugins subtree keeps only its own flags (--plugins-dir, --skip-cluster-checks).
• Error hints, help texts, and docs point to the new paths.
• No flag, alias, or behavior is lost - see Tests.

The new summary

d8 dist with no arguments prints the state of the distribution.
If the cluster is unreachable, it prints local data and a warning instead of failing:

$ d8 dist
deckhouse-cli (d8)
  Version:  v0.33.5
  Latest:   v0.34.0  update available - run 'd8 dist update'

Plugins (2 installed):
  NAME    VERSION  LATEST  STATUS
  dk      0.5.1    0.5.1   up to date
  system  1.2.0    1.3.0   update available

Update a plugin with 'd8 dist plugins update <name>' or 'd8 dist plugins update all'.

• "Latest" for the binary and for each plugin comes from the in-cluster registry-packages-proxy, same as check and plugins versions.
• The summary is read-only: it never creates the plugins directory and finds installs in the home fallback (~/.deckhouse-cli) too.

Breaking change

d8 cli ... and d8 plugins ... are gone; scripts must switch to
d8 dist ... / d8 dist plugins ....

• d8 cli shipped in releases since v0.32.0.
• d8 plugins was always hidden and undocumented.