daemon: auto-assign pilot-<nodeID> hostname when none configured

Previously, a daemon started without Config.Hostname registered with
no hostname at all — the operator had to run `pilotctl set-hostname`
before anything could resolve the node by name. Most users skipped
that step.

This change defaults to `pilot-<8 lowercase hex of nodeID>` when
Config.Hostname is empty:

- Format matches the registry's hostname regex
  `^[a-z0-9]([a-z0-9-]{0,61}[a-z0-9])?$` (14 chars, alphanumeric
  endpoints, only [a-z0-9-] inside).
- Stable: the nodeID is the daemon's persistent identity, so the
  auto-default doesn't churn across restarts.
- Unique: 32-bit nodeID is registry-allocated; collisions are
  impossible without an identity collision (which would prevent
  registration anyway).
- Operator override: `pilotctl set-hostname <name>` still wins —
  setNodeHostname is a plain swap, no special case.

The re-registration path (post-disconnect heal in pkg/daemon/daemon.go
~4249) gets the same treatment so a node's hostname survives a
registry reconnection.

Tests cover the format/stability invariant and a sweep against the
duplicated registry regex (so the test fails loudly if the registry
ever tightens validation in a way that would break the default).

Author: teovl