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 Superseded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/service/bot_command/io.rs

@@ -64,11 +64,6 @@ pub(super) async fn call_rmtree(
         .await
 }
 
-/// Call the Matrix `rebuild` handler.
-pub(super) async fn call_rebuild(project_root: &Path, agents: &Arc<AgentPool>) -> String {
-    crate::chat::transport::matrix::rebuild::handle_rebuild("web-ui", project_root, agents).await
-}
-
 /// Parse and execute a `timer` command.
 ///
 /// Returns `Err` with a usage string if the timer arguments cannot be parsed.

server/src/service/bot_command/mod.rs

@@ -83,7 +83,6 @@ pub async fn execute(
             let number = parse::parse_number("rmtree", args).map_err(Error::BadArgs)?;
             Ok(io::call_rmtree(&number, project_root, agents).await)
         }
-        "rebuild" => Ok(io::call_rebuild(project_root, agents).await),
         "timer" => io::call_timer(args, project_root)
             .await
             .map_err(Error::BadArgs),