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 Superseded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/mcp/dispatch.rs

@@ -81,8 +81,6 @@ pub async fn dispatch_tool_call(
         // Diagnostics
         "get_server_logs" => diagnostics::tool_get_server_logs(&args),
         "get_version" => diagnostics::tool_get_version(ctx),
-        // Server lifecycle
-        "rebuild_and_restart" => diagnostics::tool_rebuild_and_restart(ctx).await,
         // Permission bridge (Claude Code → frontend dialog)
         "prompt_permission" => diagnostics::tool_prompt_permission(&args, ctx).await,
         // Token usage