Adds hermes_cli/context_switch_guard.py mirroring the model_cost_guard
pattern. When a user switches models mid-session (Herm TUI picker, CLI,
or /model on Telegram/Discord), the warning surfaces on the existing
ModelSwitchResult.warning_message path used by the expensive-model
guard if the new model's compression threshold is below the current
session size.
Partial fix for #23767 — addresses only the 'user-facing guardrail
when switching from a high-context provider to a substantially
lower-context provider' slice. The other proposed fixes from that
issue (hard preflight token guard, metadata cache invalidation on
switch, compression safety invariant, oversized tool-output handling)
are out of scope for this PR.
