Remove customers: tags replaced them

[bencode]

Removes the customer grouping, which tags replaced (#6). Unlike the rest of the protocol's history this is a deliberate removal, agreed after the structural review.

Removed

• Protocol: Overview.customers, CustomerOverview, HostView.customer, CustomerId.
• Configuration: [[customers]] (ServerConfig.customers, Customer) and a host's customer, with their validation.
• Server: the per-customer grouping built in views::overview, which also had an always-empty endpoints.
• Agent: customer: None in the local status view.

Behaviour

• Old configuration. A server.toml that still has [[customers]] or a host customer fails to load:
  [[customers]] and a host's customer are no longer read: tag the hosts instead (tags = ["<customer>"]).
  The check reads only the top-level customers key and customer directly inside a host, so app, probe or mute keys are never matched. It runs after the normal parse has succeeded, so syntax errors keep their line-only, secret-free message.
• Banner. The view's "server is older than this view" banner now fires on an overview with no hosts and no problems but a status other than ok. A current server cannot produce that: with no hosts, its status is the rollup of its problems.
• Compatibility.
  • An older skym-view still parses the new overview (customers was serde(default)).
  • A new view ignores an older server's customers.
  • Agents never sent customers.

Verification

• 258 tests pass; clippy and fmt are clean.
• cargo-mutants on the diff: 7 mutants, none missed.
• The production server.toml loads with the new binary, which reached listening.
• A configuration with [[customers]] is rejected with the message above.
• An independent review found nothing material. It checked completeness, false positives and negatives of the check, the banner condition, and compatibility.

Upgrade

Server, then skym-view. Agents are unchanged. The production configuration needs no edit.