fix(talos-cluster): guard per-node hostname patch against empty/whitespace strings

[jfreed-dev]

Follow-up to #57 (per-node Talos hostnames), addressing two items found in a max-effort code review of that change.

Problem

The hostname patch from #57 guarded only on hostname != null:

config_patches = each.value.hostname != null ? [
  yamlencode({ machine = { network = { hostname = each.value.hostname } } })
] : []

An explicit empty string (e.g. hostname = var.cp_hostname where the var is unset → "") passes != null, so the module pushed machine.network.hostname: "" to the node instead of treating it as "no hostname override".

Fix

• Empty/whitespace guard (main.tf, both control-plane and worker blocks): guard on trimspace(...) != "", with try() covering the null case, and trim the value used in the patch. Blank/whitespace-only hostnames are now a no-op (Talos keeps its auto-generated name), identical to null.
• Docs — the hostname input was undocumented beyond the type:
  • New "Node Hostnames" section in the module README, including the fresh-builds-only caveat: Talos cannot rename an already-running node (static hostname already set), so a live cluster must be wiped and re-provisioned to change a hostname.
  • Expanded control_plane / workers variable descriptions (and the synced terraform-docs input table) with the same notes.
• CHANGELOG: entries under [Unreleased].

Verification

Guard truth table via terraform console:

input	try(trimspace(x), "") != ""	patch applied?
null	false	no (was: no)
""	false	no (was: yes — the bug)
" "	false	no (was: yes)
"turing-cp1"	true	yes

Also ran terraform fmt -check (clean) and terraform validate (success) on the module.

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