The dashboard form is built from CONFIG_SCHEMA, which doesn't enumerate
every root-level key the YAML supports. Most visibly, `custom_providers`
is in `_KNOWN_ROOT_KEYS` but is absent from the schema — so the frontend
never sends it in the PUT body. The previous full-replace save() then
silently wiped the key from disk every time the user clicked anything
that triggered a save. Other casualties (less visible because defaults
re-mask them on load) include `agent.personalities`,
`agent.reasoning_effort`, `terminal.lifetime_seconds`, etc.
Fix: read the raw on-disk config and deep-merge the incoming PUT body
on top of it before saving. The frontend can only overwrite what it
explicitly sends; everything else is preserved verbatim.
Reuses the existing `_deep_merge` helper from `hermes_cli.config`.
Tests:
- `test_round_trip_preserves_custom_providers` exercises the exact bug:
seed config with custom_providers, GET → drop the key → PUT,
assert it's still on disk.
- `test_round_trip_preserves_schema_invisible_nested_keys` covers the
shallow-vs-deep-merge case for nested dicts under `agent` etc.
Both fail on current main; both pass with this patch.
76af2456a2