feat(gateway-tool): add config.patch action for safe partial config updates (#1624)

* fix(ui): enable save button only when config has changes

The save button in the Control UI config editor was not properly gating
on whether actual changes were made. This adds:
- `configRawOriginal` state to track the original raw config for comparison
- Change detection for both form mode (via computeDiff) and raw mode
- `hasChanges` check in canSave/canApply logic
- Set `configFormDirty` when raw mode edits occur
- Handle raw mode UI correctly (badge shows "Unsaved changes", no diff panel)

Fixes #1609

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* feat(gateway-tool): add config.patch action for safe partial config updates

Exposes the existing config.patch server method to agents, allowing safe
partial config updates that merge with existing config instead of replacing it.

- Add config.patch to GATEWAY_ACTIONS in gateway tool
- Add restart + sentinel logic to config.patch server method
- Extend ConfigPatchParamsSchema with sessionKey, note, restartDelayMs
- Add unit test for config.patch gateway tool action

Closes #1617

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

src/gateway/protocol/schema/config.ts

@@ -27,6 +27,9 @@ export const ConfigPatchParamsSchema = Type.Object(
   {
     raw: NonEmptyString,
     baseHash: Type.Optional(NonEmptyString),
+    sessionKey: Type.Optional(Type.String()),
+    note: Type.Optional(Type.String()),
+    restartDelayMs: Type.Optional(Type.Integer({ minimum: 0 })),
   },
   { additionalProperties: false },
 );