refactor(hermes): simplify schema handling + preserve unknown provider fields

Drops the v11→v12 providers-dict compat layer: CC Switch now only
reads/writes `custom_providers:`, leaving migrated `providers:` dict
entries to Hermes Web UI for reconciliation (Hermes' runtime already
merges both shapes via `get_compatible_custom_providers`). The
`schema_migrated_v12` health warning now points users there when a
dict-migrated config is detected.

Adds forward-compat merge to `set_provider`: when updating an existing
entry, on-disk fields the UI payload didn't submit (e.g. Hermes-only
`request_timeout_seconds`, `key_env`) are carried over. Without this,
editing one field via CC Switch would silently strip the rest.

Adds `set_memory_enabled` + `set_hermes_memory_enabled` Tauri command
for the upcoming memory-switch UI. Writes go through a merge-aware
section replacement so character budgets and external-provider fields
survive toggle operations.

Removes four dict-only helpers (`normalize_providers_dict_entry_for_read`,
`rename_alias_key`, `json_obj_non_empty_str`,
`resolve_provider_name_from_yaml_entry`) and the multi-section write
helper. Simplifies `get_providers` / `remove_provider` / health scan
back to list-only. Replaces nine obsolete dict-related tests with
`set_provider_preserves_unknown_fields_on_update` and
`set_memory_enabled_preserves_other_fields`.

src-tauri/src/commands/hermes.rs

@@ -76,6 +76,14 @@ pub fn get_hermes_memory_limits() -> Result<hermes_config::HermesMemoryLimits, S
     hermes_config::read_memory_limits().map_err(|e| e.to_string())
 }
 
+#[tauri::command]
+pub fn set_hermes_memory_enabled(
+    kind: hermes_config::MemoryKind,
+    enabled: bool,
+) -> Result<hermes_config::HermesWriteOutcome, String> {
+    hermes_config::set_memory_enabled(kind, enabled).map_err(|e| e.to_string())
+}
+
 // ============================================================================
 // Hermes Web UI launcher
 // ============================================================================