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 Superseded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/chat/commands/mod.rs

@@ -224,11 +224,6 @@ pub fn commands() -> &'static [BotCommand] {
             description: "Clear the current Claude Code session and start fresh",
             handler: handle_reset_fallback,
         },
-        BotCommand {
-            name: "rebuild",
-            description: "Rebuild the server binary and restart",
-            handler: handle_rebuild_fallback,
-        },
         BotCommand {
             name: "timer",
             description: "Schedule a deferred agent start: `timer <story_id> <HH:MM>`, `timer list`, `timer cancel <story_id>`",
@@ -425,16 +420,6 @@ fn handle_reset_fallback(_ctx: &CommandContext) -> Option<String> {
     None
 }
 
-/// Fallback handler for the `rebuild` command when it is not intercepted by
-/// the async handler in `on_room_message`.  In practice this is never called —
-/// rebuild is detected and handled before `try_handle_command` is invoked.
-/// The entry exists in the registry only so `help` lists it.
-///
-/// Returns `None` to prevent the LLM from receiving "rebuild" as a prompt.
-fn handle_rebuild_fallback(_ctx: &CommandContext) -> Option<String> {
-    None
-}
-
 /// Fallback handler for the `cleanup_worktrees` command when it is not
 /// intercepted by the async handler in `on_room_message`.  In practice this is
 /// never called — cleanup_worktrees is detected and handled before