Remove all alternate update paths — fleet redeploy is release + upgrade all

Killed:
- rebuild_and_restart (in-container cargo self-compile): the MCP tool,
  the `rebuild` chat command in all four transports, the web-ui bot
  command, and the underlying function. This was the path that caused
  the exec() deadlocks.
- upgrade_sled gateway MCP tool: second entry point to sled upgrades,
  defaulted to serving the gateway's own macOS binary to Linux sleds.
- GET /api/huskies-binary (both sled and gateway route trees): served
  current_exe(), wrong platform when the gateway is macOS. Superseded
  by /api/artifacts/ which now also serves on the gateway route tree.
- `huskies upgrade` CLI subcommand and --source flag: third way of
  doing the same download-and-replace. Escape hatch for a bricked sled
  is `docker cp` + restart.

Kept, distinct jobs: `project-rebuild` (container/image updates),
`rebuild gateway` + script/local-release (gateway self-update).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019fHdm92yjvguPi2LiXfLB9

server/src/http/context.rs

@@ -57,8 +57,8 @@ pub struct AppContext {
     pub qa_app_process: Arc<std::sync::Mutex<Option<std::process::Child>>>,
     /// Best-effort shutdown notifier for active bot channels (Slack / WhatsApp).
     ///
-    /// When set, the MCP `rebuild_and_restart` tool uses this to announce the
-    /// shutdown to configured channels before re-execing the server binary.
+    /// When set, restart-inducing paths use this to announce the shutdown to
+    /// configured channels before the process exits.
     /// `None` when no webhook-based bot transport is configured.
     pub bot_shutdown: Option<Arc<BotShutdownNotifier>>,
     /// Watch sender used to signal the Matrix bot task that the server is