Adds hermes_cli/model_selection_guards.py: a single evaluation point that
runs every selection guard (cost + the new data-policy guard) and returns
the warnings that fired. All seven model-selection surfaces (CLI picker,
cli.py TUI modal, gateway typed /model, dashboard web_server, TUI gateway,
Telegram and Discord pickers) now call the registry instead of importing
model_cost_guard directly — so the data-training-tier warning from
PR #81416 fires everywhere at once, and future guards need zero surface
wiring.
Guard modules keep their public APIs; existing mock patch points
(hermes_cli.model_cost_guard.expensive_model_warning) remain valid.
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