Adds two new agent settings nested under `auto_compact`, `enabled` and
`threshold`, which control automatic compaction of the agent's context
window as part of the handoff feature. Both surface in the settings UI
under Agent Configuration, gated behind the `handoff` feature flag so
they only appear for users on that flag.

The `threshold` accepts three forms: a percentage string ending in `%`
(e.g. `"80%"` or `"95.5%"`) measured against the model's context window,
a positive integer to compact after that many tokens have been used, or
a negative integer to compact once that many tokens remain in the
window. `0` and bare non-integer numbers are rejected. It deserializes
from either a JSON string or integer and resolves to a typed enum,
falling back to the `"80%"` default (with a logged warning) when the
configured value is invalid. In the settings UI the threshold is a
free-form text field so all three forms can be entered.

Release Notes:

- N/A

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Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>