fix: the launcher follows the config the gateway applied

[mekjr1]

Fixes #4.

Gateway restart required compares the saved config's restart signature with the one the launcher believes the gateway applied (gateway.bootConfig). The launcher updated that belief only after a reload it asked for itself (applyLiveConfig). A gateway that reloaded on its own, through gateway.hot_reload or /reload in a chat, applies everything the signature covers, yet the indicator stayed until a restart.

Now the gateway says which config it applied:

• The restart signature moves from the launcher to pkg/config (RestartSignature, NewRestartSignature, ModelSelections), unchanged, so the kernel can compute it. RestartSignature.Digest identifies a signature without showing any setting.
• The gateway reports the digest of the signature of the config it applied on /ready (config_digest), at start and after each successful reload. Because the gateway computes it from its own loaded config, it covers what LoadConfig adds outside the files: the environment's overrides and file:// credentials.
• The launcher reads it from the /ready probe its status already makes. While the gateway reports one, the indicator is whether that is the saved config's, except while a reload the launcher asked for is pending, when the old comparison (live parts left out) still applies. A match also records the saved config as applied (recordAppliedConfigLocked, used by applyLiveConfig too): default model, signature, and the web chat token taken from that config, not re-read from disk. So the indicator also follows a gateway the launcher attached to, and one whose reload failed.

A reload leaves gateway.host, gateway.port and gateway.hot_reload as they were; the signature doesn't cover them. A gateway that reports no digest keeps the old behaviour.

Docs: docs/troubleshooting.md says the indicator lasts until the gateway restarts or reloads. CHANGELOG line.

Tests: TestRestartSignatureDigest, TestReadyHandler_ReportsTheAppliedConfig, and TestGatewayStatusFollowsTheConfigTheGatewayApplied. The last covers before a reload, after it, and a gateway that runs another config than the launcher took it for; it fails without the change.