Add remote plugin skill read API (#20150)

## Summary

Adds an app-server `plugin/skill/read` method for remote plugin skill
markdown. The new method calls the plugin-service skill detail endpoint
and returns `skill_md_contents`, so clients can preview skills for
remote plugins before the bundle is installed locally.

## Why

Uninstalled remote plugin skills do not have local `SKILL.md` files.
Without an on-demand remote read, the desktop plugin details UI cannot
render the skill details modal for those skills.

## Validation

- `just write-app-server-schema`
- `just fmt`
- `cargo test -p codex-app-server-protocol`
- `cargo test -p codex-app-server --test all --
suite::v2::plugin_read::plugin_skill_read_reads_remote_skill_contents_when_remote_plugin_enabled
--exact`
- `just fix -p codex-app-server-protocol -p codex-core-plugins -p
codex-app-server`

sdk/python/src/codex_app_server/generated/notification_registry.py

@@ -38,6 +38,7 @@ from .v2_all import PlanDeltaNotification
 from .v2_all import ReasoningSummaryPartAddedNotification
 from .v2_all import ReasoningSummaryTextDeltaNotification
 from .v2_all import ReasoningTextDeltaNotification
+from .v2_all import RemoteControlStatusChangedNotification
 from .v2_all import ServerRequestResolvedNotification
 from .v2_all import SkillsChangedNotification
 from .v2_all import TerminalInteractionNotification
@@ -100,6 +101,7 @@ NOTIFICATION_MODELS: dict[str, type[BaseModel]] = {
     "mcpServer/startupStatus/updated": McpServerStatusUpdatedNotification,
     "model/rerouted": ModelReroutedNotification,
     "model/verification": ModelVerificationNotification,
+    "remoteControl/status/changed": RemoteControlStatusChangedNotification,
     "serverRequest/resolved": ServerRequestResolvedNotification,
     "skills/changed": SkillsChangedNotification,
     "thread/archived": ThreadArchivedNotification,