Fixes #12293.

In list_providers(), Section 3 (user-defined endpoints) appends each provider to results but never registers its slug in seen_slugs. Section 4 (saved custom providers) generates the custom:-prefixed slug via custom_provider_slug() and checks seen_slugs for deduplication — but since the plain ep_name was never registered, the check never fires and the same provider is emitted twice in /model.

Root cause

hermes_cli/model_switch.py, Section 3:

results.append({
    "slug": ep_name,
    ...
})
# ep_name never added to seen_slugs — Section 4 can't dedup against it

Section 4 generates custom:<name> via custom_provider_slug(), checks seen_slugs, finds nothing, and emits a duplicate.

Fix

After the results.append() in Section 3, register both slug forms:

seen_slugs.add(ep_name)
seen_slugs.add(custom_provider_slug(display_name))

This ensures Section 4's dedup check correctly suppresses the duplicate entry regardless of which slug format it generates. custom_provider_slug is already imported at module level (line 28) — no new imports required.

Test plan

• Define a custom provider in ~/.hermes/config.yaml that also appears in the built-in registry cache
• Run /model — verify the provider appears exactly once
• Verify providers that exist only in Section 3 or only in Section 4 still appear normally